System and methods for objective evaluation of hearing using auditory steady-state responses

ABSTRACT

This invention relates to an apparatus and method for assessing a subject&#39;s hearing by recording steady-state auditory evoked responses. The apparatus generates a steady-state auditory evoked potential stimulus, presents the stimulus to the subject, senses potentials while simultaneously presenting the stimulus and determines whether the sensed potentials contain responses to the stimulus. The stimulus may include an optimum vector combined amplitude modulation and frequency modulation signal adjusted to evoke responses with increased amplitudes, an independent amplitude modulation and frequency modulation signal and a signal whose envelope is modulated by an exponential modulation signal. The apparatus is also adapted to reduce noise in the sensed potentials by employing sample weighted averaging. The apparatus is also adapted to detect responses in the sensed potentials via the Phase weighted T-test or Phase zone technique. The apparatus may further perform a number of objective audiological tests including latency tests, AM/FM discrimination tests, rate sensitivity tests, aided hearing tests, depth sensitivity tests, supra-threshold tests and auditory threshold tests. The apparatus is further adapted to perform multi-modality testing in which more than one sensory modality of the subject is tested simultaneously.

This application claims the benefit of Provisional Applications Ser. No.60/205,469, filed May 19, 2000, Ser. No. 60/247,999, filed Nov. 14, 2000and Ser. No. 60/287,387, filed May 1, 2001.

FIELD OF THE INVENTION

This invention is in the field of auditory assessment and relates to theidentification and evaluation of hearing impairment. More particularly,the invention describes a system and method for objectively evaluatingan individual's hearing abilities by recording auditory steady-stateresponses.

BACKGROUND OF THE INVENTION

Hearing-impairment is a significant health problem, particularly at thetwo ends of the human life span. Approximately one in a thousand newborninfants and more than a quarter of adults over the age of 65 have asignificant hearing loss. In the case of infants, early detection ofhearing loss is necessary to ensure that appropriate treatment isprovided at an early stage and that the infant can develop normal speechand language. The detection of a hearing-impairment requires ameasurement of how well someone can hear sound (i.e. audiometry).

Conventional audiometry is performed by having a subject respond toacoustic stimuli by pressing a button, saying “yes”, or repeating wordsthat may be presented in the stimulus. These tests are subjective innature. Audiometry allows an audiologist to determine the auditorythreshold of the subject, which is defined as the lowest intensity atwhich a sound can be heard. The audiologist evaluates the auditorythreshold of a subject by using a stimulus that most commonly consistsof a pure tone. The stimulus is presented via earphones, headphones,free field speakers or bone conduction transducers. The results arepresented as an audiogram which shows auditory thresholds for tones ofdifferent frequencies. The audiogram is helpful for diagnosing the typeof hearing loss a subject may have. The audiogram can also be used tofit a hearing aid and adjust the level of amplification of the hearingaid for subjects who require hearing aids.

Audiometry may also involve subjective testing at supra-thresholdintensities to determine how well the subject's auditory systemdiscriminates between different sounds (such as speech) presented atintensities at which they normally occur. The audiologist will thereforedetermine how many simple words a subject can accurately recognize atdifferent intensities with and without different amounts of backgroundnoise. The audiologist may also conduct tests which measure how well thesubject can discriminate changes in the intensity or frequency of asound or how rapidly these changes occur.

Conventional audiometry cannot be performed if the subject is an infant,young child or cognitively impaired adult. In these cases, objectivetests of hearing are necessary in which the subject does not have tomake a conscious response. Objective audiometry is essential fordetecting hearing impairment in infants or elderly patients as well asfor evaluating functional hearing losses. Furthermore, few objectivetests have been developed for supra-threshold tests of speech,frequency, or intensity discrimination.

One form of objective audiometry uses auditory evoked potentials.Auditory evoked potential testing consists of presenting the subjectwith an acoustic stimulus and simultaneously or concurrently sensing(i.e. recording) potentials from the subject. The sensed potentials arethe subject's electroencephalogram (EEG) which contain the subject'sresponse to the stimulus if the subject's auditory system has processedthe stimulus. These potentials are analyzed to determine whether theycontain a response to the acoustic stimulus or not. Auditory evokedpotentials have been used to determine auditory thresholds and hearingat specific frequencies.

One particular class of auditory evoked potentials is steady-stateevoked potentials (SSAEPs). The stimulus for the SSAEP consists of acarrier signal, which is usually a sinusoid, that is amplitude modulatedby a modulation signal which is also usually a sinusoid. The SSAEPstimulus is presented to the subject while simultaneously recording thesubject's EEG. If the auditory system of the subject responded to theSSAEP stimulus, then a corresponding steady-state sinusoidal signalshould exist in the recorded EEG. The signal should have a frequencythat is the same as the frequency of the modulation signal (i.e.modulation frequency). The presence of such a corresponding signal inthe EEG is indicative of a response to the SSAEP stimulus.Alternatively, the phase of the carrier signal may be frequencymodulated instead of or in addition to amplitude modulation to createthe SSAEP stimulus.

The SSAEP stimulus is sufficiently frequency-specific to allow aparticular part of the auditory system to be tested. Furthermore, theSSAEP stimulus is less liable to be affected by distortion in free-fieldspeakers or hearing aids. Typical modulation frequencies which are usedin SSAEP stimuli are between 30 to 50 Hz or 75 to 110 Hz. The latterrange may be particularly useful for audiometry because at these rates,the SSAEP responses are not significantly affected by sleep and can bereliably recorded in infants. Furthermore, SSAEP responses at theserates result in audiometric threshold estimates that are well correlatedwith behavioral thresholds to pure tone stimuli. In SSAEP testing, thepresence or absence of an SSAEP response to an SSAEP stimulus can bedetermined using several statistical techniques.

However, objective audiometry employing SSAEP testing is time-consumingbecause the amplitude of the SSAEP response is quite small compared tothe background noise which is the subject's ongoing brain activity (i.e.EEG) while the test is being conducted. The SSAEP response thus has asmall signal-to-noise ratio (SNR) which makes it difficult to detect theSSAEP response in a short time period. One technique to reduce SSAEPtesting time is to use a multiple SSAEP stimulus which combines severalSSAEP test signals (i.e. where a test signal is meant to mean one SSAEPstimulus). The potentials sensed from the subject during thepresentation of the multiple SSAEP stimulus contains a linearsuperposition of SSAEP responses to each SSAEP test signal in themultiple SSAEP stimulus. This makes it possible to record the SSAEPresponses to multiple (e.g., four or eight) stimuli in the same timethat it takes to record the response to a single stimulus. Therefore,this technique results in a reduction of test time since the SSAEPresponses to several SSAEP test signals may be detected concurrently.However, the SNR for each SSAEP response is still small and the testingtime for recording the response to a single SSAEP stimulus has not beenreduced. To reduce the SSAEP test time techniques are required to eitherincrease the amplitude of the SSAEP response and/or decrease theamplitude of the noise that is recorded along with the SSAEP response. Amore sensitive statistical method that can detect SSAEP responses withsmall SNRs would also be useful.

While objective testing identifies that a subject has a hearing loss,the next step is usually to treat the subject by providing them with ahearing aid. However, if the subject is an infant, a method is requiredto objectively adjust the hearing aid since this cannot be done withconventional subjective methods. Some objective methods have beendeveloped such as determining the real-ear insertion gain when a hearingaid is in place. However, this method is only useful if one knows theactual unaided audiometric thresholds of the subject so that the hearingaid can be adjusted to match prescriptive targets. Furthermore,placement of a probe-tube in an infant can be challenging. There havealso been methods based on click evoked auditory evoked potentials (i.e.wave V of the click-evoked ABR) but the stimuli used in these methodsare restricted to certain frequency ranges and do not test the abilityof the hearing aid to process continuous signals like speech.Accordingly, there still remains a need for an objective method tomeasure the benefits of a hearing aid in patients where behavioralthresholds and real-ear measurements are difficult to obtain.

SUMMARY OF THE INVENTION

The present invention is an apparatus for recording SSAEP responses anda set of methods for using the apparatus to test various aspects of asubject's auditory system. The apparatus comprises hardware to presentSSAEP stimuli, to acquire EEG data while simultaneously presenting theSSAEP stimuli, and to analyze the EEG data to detect the presence ofSSAEP responses. The apparatus further comprises software to enable thecreation and presentation of the SSAEP stimuli, the acquisition of theEEG data (i.e. electrophysiological potentials) and the analysis of theEEG data. The software further enables displaying the results of ongoingtesting and the final results of the test as well as the storage of thetest results for subsequent viewing and/or analysis.

The present invention also includes software adapted to effect noisereduction algorithms which may include sample weighted averaging. Thesoftware is also adapted to effect statistical tests that are used todetect the SSAEP responses to the SSAEP stimuli. These statistical testsmay include the phase weighted t-test, the phase zone technique and themodified Rayleigh test of circular uniformity (MRC).

The present invention also uses particular types of SSAEP stimuli toincrease the amplitude of the resulting SSAEP responses. These SSAEPstimuli may include a combined amplitude modulation and frequencymodulation signal in which the phase of the frequency modulated signalis adjusted relative to the phase of the amplitude modulated. TheseSSAEP stimuli may also include using an exponential modulation signal.The present invention also uses an SSAEP stimulus consisting of anindependent amplitude modulation signal and frequency modulation signalwherein the AM modulation rate is different than the FM modulation rate.This stimulus evokes two SSAEP responses that can be independentlyanalyzed.

In another aspect of the invention, these SSAEP stimuli can be used fora variety of objective tests such as determining audiometric thresholdsand testing the aided and unaided hearing of a subject. The presentinvention further comprises several other audiometric protocolsincluding latency tests, AM/FM discrimination tests, rate sensitivitytests, aided hearing tests, depth sensitivity tests and supra-thresholdtests.

The present invention further comprises databases of normative datawhich can be used to construct SSAEP stimuli, detect SSAEP responses anddetermine whether detected SSAEP responses are indicative of normal orabnormal hearing. The databases contain data which are grouped bysubject characteristics such as age, sex and state over a variety ofstimulus characteristics such as type of SSAEP stimulus, the type ofmodulation (amplitude versus frequency), the modulation rates andmodulation depth. The database also preferably contains data about SSAEPresponse characteristics such as latency and ratio of amplitudes ofSSAEP responses to amplitude modulated and frequency modulated SSAEPstimuli.

In an alternative embodiment, the apparatus is adapted to performmulti-modality testing in which more than one sensory modality (i.e.vision and audition) of the subject is tested simultaneously.

The invention comprises a method of testing the hearing of a subjectcomprising the steps of:

(a) selecting at least one test signal;

(b) modulating at least one of the amplitude and frequency of the atleast one test signal by an exponential modulation signal to produce atleast one modulated test signal;

(c) transducing the at least one modulated test signal to create anacoustic stimulus and presenting the acoustic stimulus to the subject;

(d) sensing potentials from the subject while substantiallysimultaneously presenting the acoustic stimulus to the subject; and,

(e) analyzing the potentials to determine whether the potentialscomprise data indicative of the presence of at least one steady-stateresponse to the acoustic stimulus.

The invention further comprises testing the hearing of a subjectcomprising the steps of:

(a) creating an optimum-vector mixed modulation test signal comprisingat least one signal having an amplitude modulated component with a firstphase and a frequency modulated component with a second phase whereinthe second phase is adjusted relative to the first phase to evoke anincreased response from the subject;

(b) transducing the test signal to create an acoustic stimulus andpresenting the acoustic stimulus to the subject;

(c) sensing potentials from the subject while substantiallysimultaneously presenting the acoustic stimulus to the subject; and,

(d) analyzing the potentials to determine whether the potentialscomprise data indicative of the presence of at least one steady-stateresponse to the acoustic stimulus.

In another aspect, the invention comprises a method for testing thehearing of a subject comprising the steps of:

(a) creating a test signal comprising at least one independent amplitudemodulated and frequency modulated signal having an amplitude modulatedcomponent and a frequency modulated component, wherein the amplitudemodulated component comprises a first modulation frequency and a firstcarrier frequency and the frequency modulated component comprises asecond modulation frequency and a second carrier frequency wherein thefirst modulation frequency is substantially different from the secondmodulation frequency and the first carrier frequency is substantiallysimilar to the second carrier frequency;

(b) transducing the test signal to create an acoustic stimulus andpresenting the acoustic stimulus to the subject;

(c) sensing potentials from the subject while substantiallysimultaneously presenting the acoustic stimulus to the subject; and,

(d) analyzing the potentials to determine whether the potentialscomprise data indicative of a steady-state response to each amplitudemodulated component and a steady-state response to each frequencymodulated component.

In another aspect, the invention comprises an apparatus for testing thehearing of a subject, wherein the apparatus comprises:

(a) a signal creator adapted to create a test signal comprising at leastone combined amplitude modulated and frequency modulated signal havingan amplitude modulated component with a first phase and a frequencymodulated component with a second phase wherein the signal creatorcomprises means for adjusting the second phase relative to the firstphase;

(b) a transducer electrically coupled to the processor and adapted totransduce the test signal to create an acoustic stimulus and present theacoustic stimulus to the subject;

(c) a sensor adapted to sense potentials from the subject while theacoustic stimulus is substantially simultaneously presented to thesubject; and,

(d) a processor electrically coupled to the sensor and adapted toreceive the potentials and analyze the potentials to determine if thepotentials comprise data indicative of at least one response to theacoustic stimulus.

In another aspect, the invention comprises a method of analyzingpotentials to determine whether the potentials comprise data indicativeof the presence of at least one steady-state response to a steady-stateevoked potential stimulus. The method comprises the steps of:

(a) presenting an evoked potential stimulus to a subject;

(b) sensing potentials from the subject while substantiallysimultaneously presenting the stimulus to the subject to obtain aplurality of data points;

(c) transforming the plurality of data points into a second plurality ofdata points;

(d) biasing the second plurality of data points with an expected phasevalue to obtain a plurality of biased data points; and,

(e) applying a statistical test to the plurality of biased data pointsto detect the response.

The invention further comprises a method of detecting a response to anevoked potential stimulus comprising the steps of:

(a) presenting an evoked potential stimulus to a subject;

(b) sensing potentials from the subject while substantiallysimultaneously presenting the stimulus to the subject to obtain aplurality of data points; and,

(c) calculating phase values for the plurality of data points,

wherein, a response is detected if an adequate number of the calculatedphase values fall within a predetermined phase value range.

The invention further comprises an apparatus for testing the hearing ofa subject, wherein the apparatus comprises:

(a) a signal creator adapted to create a test signal;

(b) a transducer electrically coupled to the signal creator and adaptedto transduce the test signal to create an acoustic stimulus and presentthe acoustic stimulus to the subject;

(c) a sensor adapted to sense potentials from the subject while theacoustic stimulus is substantially simultaneously presented to thesubject; and,

(d) a processor electrically coupled to the sensor and adapted toreceive the potentials and analyze the potentials to determine if thepotentials comprise data indicative of at least one response to theacoustic stimulus;

wherein the analysis involves biasing the potentials based on anexpected phase value. The apparatus further comprises a database ofexpected phase value data correlated to subject characteristics andstimulus characteristics.

In yet another aspect, the invention comprises a method of noisereduction for a plurality of data points which are obtained duringsteady-state evoked potential testing, wherein the plurality of datapoints comprise at least one signal and noise and wherein said methodcomprises the steps of:

(a) obtaining said plurality of data points;

(b) separating said plurality of data points into a plurality of epochs;and,

(c) applying an adaptive noise reduction method to each epoch.

In another aspect, the invention comprises a method of objectivelytesting the hearing of a subject comprising the steps of:

(a) selecting an auditory test;

(b) creating an appropriate test signal comprising at least onecomponent for the auditory test;

(c) transducing the test signal to create a stimulus and presenting saidstimulus to the subject;

(d) sensing potentials from the subject while substantiallysimultaneously presenting the stimulus to the subject; and,

(e) analyzing the potentials to detect at least one response.

In another aspect, the invention further comprises an apparatus forobjectively testing the hearing of a subject, wherein the apparatuscomprises:

(a) a selector adapted for selecting an auditory test to perform on thesubject;

(b) a signal creator electrically coupled to the selector and adapted tocreate an appropriate test signal comprising at least one component forthe test;

(c) a transducer electrically coupled to the signal creator and adaptedto transduce the test signal to create an acoustic stimulus and presentthe acoustic stimulus to the subject;

(d) a sensor adapted to sense potentials from the subject while theacoustic stimulus is substantially simultaneously presented to thesubject;

(e) a processor electrically coupled to the sensor and adapted toreceive the potentials and analyze the potentials to determine if thepotentials comprise data indicative of at least one response to theacoustic stimulus; and,

(f) a programmable hearing aid coupled to said processor, wherein, theprogrammable hearing aid comprises a plurality of programmable gainfactors for different frequency regions and at least one programmablefilter slope.

In an alternative embodiment, the invention comprises a method oftesting at least two senses of a subject, wherein the method comprisesthe steps of:

(a) selecting a first steady-state test signal to test a first sensorymodality;

(b) transducing the first steady-state test signal to create a firststimulus and presenting the first stimulus to the subject;

(c) selecting a second steady-state test signal to test a second sensorymodality;

(d) transducing the second steady-state test signal to create a secondstimulus and presenting the second stimulus to the subject;

(e) sensing potentials while substantially simultaneously presentingboth stimuli to the subject; and,

(f) analyzing the potentials to determine whether the potentialscomprise data indicative of at least one steady-state response to thestimuli.

In an alternative embodiment, the invention further comprises anapparatus for testing at least two senses of a subject, wherein theapparatus comprises:

(a) a signal creator adapted to create a first steady-state test signaland a second steady-state test signal;

(b) a first transducer electrically coupled to the selector and adaptedto transduce the first test signal to create a first stimulus andpresent the first stimulus to the subject;

(c) a second transducer electrically coupled to the selector and adaptedto transduce the second test signal to create a second stimulus andpresent the second stimulus to the subject;

(d) a first sensor adapted to sense first potentials from the subjectwhile the first stimulus is substantially simultaneously presented tothe subject;

(e) a second sensor adapted to sense second potentials from the subjectwhile the second stimulus is substantially simultaneously presented tothe subject;

(f) a processor electrically coupled to the first sensor and adapted toreceive the first potentials and analyze the first potentials todetermine if the first potentials comprise data indicative of at leastone response to the first stimulus; and,

(g) the processor, electrically coupled to the second sensor and adaptedto receive the second potentials and analyze the second potentials todetermine if the second potentials comprise data indicative of at leastone response to the second stimulus,

wherein, each stimulus is presented substantially simultaneously.

Further objects and advantages of the invention will appear from thefollowing description, taken together with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the present invention and to show moreclearly how it may be carried into effect, reference will now be made,by way of example, to the accompanying drawings which show a preferredembodiment of the present invention and in which:

FIG. 1a is a schematic of an embodiment of the apparatus of the presentinvention;

FIG. 1b is a flow diagram illustrating the general objective auditorytest methodology;

FIG. 2a is a histogram of EEG amplitudes during a noisy recordingsession;

FIG. 2b is a histogram of EEG amplitudes during a quiet recordingsession;

FIG. 2c is the amplitude spectrum of the result of performing normaltime averaging on the EEG data shown in FIG. 2a;

FIG. 2d is the amplitude spectrum of the result of performing normaltime averaging on the EEG data shown in FIG. 2b;

FIG. 2e is the amplitude spectrum of the result of performing sampleweighted averaging on the EEG data shown in FIG. 2a;

FIG. 2f is the amplitude spectrum of the result of performing sampleweighted averaging on the EEG data shown in FIG. 2b;

FIG. 2g is the amplitude spectrum of the result of performing amplituderejection on the EEG data shown in FIG. 2a and then performing normaltime averaging;

FIG. 2h is the amplitude spectrum of the result of performing amplituderejection on the EEG data shown in FIG. 2b and then performing normaltime averaging;

FIG. 3a is a plot of the results of the F-test on an averaged sweep ofEEG data points containing an SSAEP response;

FIG. 3b is a plot of the results of the phase weighted t-test for thesame set of data shown in FIG. 3a;

FIG. 4a is a graph of SSAEP response amplitudes to an MM SSAEP stimulusand an AM SSAEP stimulus for a 50 dB SPL stimulus intensity and afrequency modulation depth of 25%;

FIG. 4b is a graph of SSAEP response amplitudes to an MM SSAEP stimulusand an AM SSAEP stimulus for a 40 dB SPL stimulus intensity and afrequency modulation depth of 25%;

FIG. 4c is a graph of SSAEP response amplitudes to an MM SSAEP stimulusand an AM SSAEP stimulus for a 30 dB SPL stimulus intensity and afrequency modulation depth of 25%;

FIG. 4d is a graph of SSAEP response amplitudes to an MM SSAEP stimulusand an AM SSAEP stimulus for a 50 dB SPL stimulus intensity and afrequency modulation depth of 10%;

FIG. 4e is a graph of SSAEP response amplitudes to an MM SSAEP stimulusand an AM SSAEP stimulus for a 40 dB SPL stimulus intensity and afrequency modulation depth of 10%;

FIG. 4f is a graph of SSAEP response amplitudes to a MM SSAEP stimulusand an AM SSAEP stimulus for a 30 dB SPL stimulus intensity and afrequency modulation depth of 10%;

FIG. 5a is a plot illustrating how the response to an SSAEP stimuluscontaining amplitude modulated and frequency modulated components can bemodeled as the vector addition of the SSAEP response to the amplitudemodulated component of the SSAEP stimulus and the SSAEP response to thefrequency modulated component of the SSAEP stimulus;

FIG. 5b is a graph illustrating how the response to an SSAEP stimuluscontaining amplitude modulated and frequency modulated components can bemodeled as a sinusoid when the phase of the frequency modulatedcomponent of the SSAEP stimulus is varied with respect to the phase ofthe amplitude modulated component of the SSAEP stimulus;

FIG. 6a is the amplitude spectrum of the SSAEP response to an AM SSAEPstimulus, an FM SSAEP stimulus and an IAFM SSAEP stimulus;

FIG. 6b is a group of polar plots showing the detection of the SSAEPresponses shown in FIG. 6a;

FIG. 7a is a graph of test results showing percent increase in SSAEPresponses when using an exponential modulation signal in the SSAEPstimulus as compared to an AM SSAEP stimulus for a stimulus intensity of50 dB pSPL;

FIG. 7b is a graph of test results showing percent increase in SSAEPresponses when using an exponential modulation signal in the SSAEPstimulus as compared to an AM SSAEP stimulus for a stimulus intensity of30 dB pSPL;

FIG. 7c is a graph of test results showing amplitudes of SSAEP responseswhen using an exponential modulation signal in the SSAEP stimulus ascompared to an AM SSAEP stimulus for a stimulus intensity of 50 dB pSPL;

FIG. 7d is a graph of test results showing amplitudes of SSAEP responseswhen using an exponential modulation signal in the SSAEP stimulus ascompared to an AM SSAEP stimulus for a stimulus intensity of 30 dB pSPL;

FIG. 8 is a pair of graphs of the latencies calculated for 80 and 160 Hzmodulation rates for SSAEP responses in response to SSAEP stimulipresented to the right and left ears of a group of subjects;

FIG. 9 is a plot of word discrimination as a function of the number ofsignificant responses to IAFM SSAEP stimuli for various subjects;

FIG. 10 is a graph of amplitude of SSAEP responses as a function ofSSAEP stimulus modulation rate for a group of young control subjects andfor an older subject with minor hearing loss;

FIG. 11 is a schematic diagram of how an audiometric threshold can beestimated using an algorithm that automatically adjusts sound intensityon the basis of whether an SSAEP response is detected;

FIG. 12 is a schematic of an objective multi-modality test apparatus;and,

FIG. 13 is a schematic of a portable version of the objectiveaudiometric test apparatus which is also adapted to performmulti-modality testing.

DETAILED DESCRIPTION OF THE INVENTION

The present invention provides an apparatus for recording steady-stateevoked potentials and a set of methods for using the apparatus to testvarious aspects of a subject's hearing. The basic hardware and softwarecomponents of the apparatus will be discussed first. Noise reductionmethods will be discussed next followed by response detection. Testsignals which can be used for SSAEPs will then be discussed. Finally,protocols for objective audiometric testing based on SSAEP stimuli willbe discussed.

Hardware and Software Components of the Invention

Referring to FIG. 1a, the objective audiometric test apparatus 10includes a processor 12, a data acquisition board 14 having a digital toanalog converter (DAC) 16 and an analog to digital converter (ADC) 18,an audiometer 20 having a filter 22 and an amplifier 24, a transducer26, a sensor 28, a second amplifier 30, a second filter 32, a masterdatabase 52 having a plurality of databases D1, D2, to Dn, a storagedevice 34 and a display monitor 36. The processor 12 is suitablyprogrammed with a software program 40 comprising a signal creator module42, a modulator module 44 and an analysis module 46 having a noisereduction module 48 and a detection module 50.

A personal computer, for example a Pentium 750 running Windows 98, mayprovide the processor 12, storage device 34 and display monitor 36. Thesoftware program 40 is run on the personal computer and the masterdatabase 52 along with the plurality of databases D1 to Dn can be storedin the memory of the personal computer and can communicate with thesoftware program. Alternatively, these components may be effected on alaptop, a handheld computing device, such as a palmtop, or a dedicatedelectronics device.

The objective audiometric test apparatus 10 can be used to assess theauditory system of a subject 60 by presenting SSAEP stimuli to thesubject 60. While the stimulus is being presented, the objectiveaudiometric test apparatus 10 records sensed potentials (i.e. EEG data)and amplifies the EEG data. This is done while substantiallysimultaneously presenting the SSAEP stimuli to the subject 60. The EEGdata is then processed and statistically evaluated to determine if therecorded EEG data contains SSAEP responses. For example, data processingmay show the responses that are statistically significantly differentthan the background EEG noise levels. The design of the objectiveaudiometric test apparatus 10 follows clear principles concerning thegeneration of the acoustic stimuli, the acquisition of artifact-freedata, the analysis of EEG data in the frequency-domain and the objectivedetection of SSAEP responses in noise.

The processor 12 may be any modern processor such as a Pentium 750. Thedata acquisition board 14 is a commercial data acquisition board(AT-MIO-16E-10) available from National Instruments. Alternatively,another data acquisition board with a suitable number of input andoutput channels may be used. The data acquisition board 14 allows forthe output of data via the DAC 16 as well as the input of data via theADC 18.

The output from the DAC 16 is sent to the audiometer 20 which may alsobe under the control of the processor 12. The audiometer 20 acts tocondition the stimulus which is presented to the subject 60 via thefilter 22 and the amplifier 24. Rather than using the audiometer 20,functionally similar amplifying/attenuating and filtering hardware canbe incorporated into the audiometric test apparatus 10 to control theintensity and frequency content of the stimulus that will be presentedto the subject 60.

The SSAEP stimulus is presented to the subject 60 via the transducer 26which may be a pair of speakers, headphones or at least one insertearphone. The insert earphones may be earphones designed by EtymoticsResearch. The transducer 26 allows the SSAEP stimulus to be presented tothe left and/or right ears of the subject 60. The stimuli may also bepresented using free-field speakers, bone conduction vibrators or otheracoustic transducers.

While the stimulus is being presented to the subject 60, the EEG issubstantially simultaneously sensed from the subject 60 using the sensor28 which is typically electrodes. The electrodes generally include oneactive electrode placed at the vertex of the subject 60, one referenceelectrode placed on the neck of the subject 60 and a ground electrodeplaced at the clavicle of the subject 60. Other configurations for theelectrodes are possible. It may also be possible to use more electrodes.

The sensed EEG data is then sent to the amplifier 30 which amplifies thesensed EEG data to a level that is appropriate for the input range ofthe ADC 18. The amplifier 30 may use a gain of 10,000. The amplifiedsensed EEG data is then sent to the filter 32 which filters theamplified sensed EEG data such that sampling can be done withoutaliasing by the ADC 18. The filter 32 may have a lowpass setting of 300Hz and a highpass setting of 1 Hz. The ADC 18 receives the filteredamplified EEG data and samples this data at a rate of approximately 1000Hz. The sampling rate depends on the settings of the filter 32. Othersampling rates may also be used, however, provided that the Nyquist rateis not violated as is well understood by those skilled in the art.

The objective audiometric test apparatus 10 shown in FIG. 1 a may beextended to comprise other circuitry such as attenuation circuits whichmay be used in the calibration of the apparatus. Other circuitry may beadded to the objective audiometric test apparatus 10, to effect otheraudiometric tests such as performing an aided hearing test in which thesubject 60 has at least one hearing aid and the objective audiometrictest apparatus 10 is adapted to alter the gain of the hearing aid sothat the subject 60 can hear the SSAEP stimuli which are presented tothe subject 60 via free-field speakers.

The objective audiometric test apparatus 10 can clearly be embodied invarious ways. For example, many different types of computers may beused. In addition, the data-acquisition board 14 may have both multipleinputs (ADCs) and multiple outputs (DACs). Multiple inputs may be usedwhen SSAEP responses are recorded at multiple electrode sites on thesubject 60 and the EEG data obtained from these electrode sites are usedto increase the SNR of the SSAEP responses. Furthermore, principalcomponent analysis, or source analysis, may be used, where the varianceof the EEG data points are projected onto at least one dipole source anddata not related to the dipole source are removed from the EEG data,thereby separating signal from noise. Multiple outputs may also be used(e.g. 8 DACs) to create the acoustic stimuli that are presented to eachear of the subject 60. This would allow some components of the SSAEPstimulus to be manipulated independently from the others. For example,multiple DACs may allow a band-limited noise-masker to be increased inintensity when the SSAEP response to a particular SSAEP stimulus,presented through one of the DAC channels, becomes significant.

The software program 40, also known as the MASTER (Multiple AuditorySteady-State Response) program allows a user to select a particularauditory test to perform on the subject 60. The software program 40 ispreferably programmed using the LabVIEW™ software package available fromNational Instruments, but could be instantiated with other softwarepackages. The software program 40 comprises a plurality of modules whichare not all shown in FIG. 1a to prevent cluttering the Figure. Thesoftware program 40 controls test signal generation via the signalcreator module 42 and the modulator module 44. The software program 40also allows the operator to select from a number of objectiveaudiometric tests which will be discussed later in more detail. Thesignal creator module 42 creates data series for the carrier signalsthat are used in the SSAEP stimulus. The signal creator module 42 willtypically employ the modulator module 44 to amplitude modulate and/orfrequency modulate these carrier signals. The software program 40 thencontrols analog to digital conversion and digital to analog conversionaccording to the protocol of the auditory test that is being performed.

The software program 40 then analyzes the sensed EEG data via theanalysis module 46 which includes the noise reduction module 48 and theresponse detection module 50. As previously described, the SNR of theSSAEP response is quite small. Therefore, the sensed EEG data must beprocessed to reduce background noise. Accordingly, the noise reductionmodule 48 may employ sample weighted averaging, time averaging and/oradaptive artifact rejection (which will all be described later in moredetail). The reduced noise signal is then sent to the detection module50 to determine whether at least one SSAEP response is present withinthe data. The detection module 50 may employ the phase weighted t-test,the phase zone technique or the MRC method which will later be describedin more detail.

The software program 40 can also display test results in the frequencydomain on the display monitor 36. The software program 40 can also savethe test results on the storage device 34 which may be a hard drive orthe like for further extensive analysis by other programs. The softwareprogram 40 also allows the test results to be printed by a printer (notshown).

The software program 40 also communicates with the master database 52which comprises a plurality of databases D1 to Dn. Only a few of thesedatabases have been shown in FIG. 1 for reasons of clarity. Thedatabases contain normative data from sample populations of subjectsrelating to a variety of parameters for SSAEP testing. For instance, thedatabases include normative phase data which can be used to create anoptimal vector SSAEP stimuli having amplitude and frequency modulatedcomponents which are adjusted to evoke SSAEP responses with increasedamplitudes. The databases further contain information about theamplitude of SSAEP responses to various SSARP stimuli.

The software program 40 implements a graphical user interface whichconsists of a series of interactive screens. These interactive screensallow a user to control the software program 40, perform a desiredauditory test and analyze test results. The interactive screens comprisea Main screen, a Stimulus Set-Up screen, a View Stimulus screen, aRecording Parameters screen, a Record Data screen, a Process Data screenand several Test Result summary screens.

The Main screen permits the user to select a particular audiometric testto perform. The Main screen also allows the user to navigate through thevarious other screens that are available.

The Stimulus Set-Up screen permits the user to define up to 8 SSAEP testsignals which can be combined in a multiple SSAEP stimulus that can bepresented to both ears of the subject 60. Other embodiments of theinvention will allow more than eight stimuli to be presented. Forexample, eight stimuli may be presented to each of the two ears of thesubject 60 for a total of 16 stimuli. The user can define the frequencyof the carrier signal (i.e. carrier frequency), the frequency of themodulation signal (i.e. modulation frequency), the amplitude modulationdepth, the frequency modulation depth, the stimulus intensity, and thephase of the frequency modulation component relative to the phase of theamplitude modulation component for a particular type of SSAEP stimulus.The Stimulus Setup-Up screen therefore allows the user to choose theSSAEP stimulus to comprise an amplitude modulation test signal (AM), afrequency modulation test signal (FM), a combined amplitude modulationand frequency modulation test signal (referred to as mixed modulation orMM), an optimum vector combined amplitude modulation and frequencymodulation test signal (OVMM) and an independent amplitude modulationand frequency modulation test signal (IAFM). Furthermore, the user canalso define the envelope of the carrier signal by choosing a particularmodulation signal. In particular, the user can choose a sinusoidalsignal as the modulation signal or an exponential modulation signal asthe modulation signal. Some exemplary exponential modulation signalsinclude a sinusoid that has an exponent of 2, 3, 4 or 5. Alternatively,fractional exponents may be used.

Once, the carrier frequency and modulation frequency are chosen, thesignal creator 42 automatically adjusts these frequencies to ensure thatan integer number of cycles of the carrier signal and modulation signalcan fit in the output buffer of the DAC 16 and the input buffer of theADC 18 This is important to avoid spectral spreading in the generatedacoustic stimulus as well as to avoid spectral spreading in the sensedEEG data which are digitized by the ADC 18. The signal creator 42 mayalso be used to present test signals to the subject 60 with constantpeak-to-peak amplitudes or constant RMS amplitudes, whereby theamplitude of the envelope of the test signal is increased to compensatefor the modulation depth.

The signal creator 42 can also generate stimuli consisting of tones,broad-band noise, high-pass noise, low-pass noise, or band-pass noiseall of which can be either modulated or unmodulated. In the case ofnoise, the signal creator 42 may allow the user to adjust the band-passand band-stop characteristics of the noise including the roll-off of thetransition region that is between the band-pass and band-stop regions.Currently the objective audiometric test apparatus 10 uses a circularbuffer in the DAC 16. However, in the case of noise stimuli, theincorporation of a double buffering technique may be used where data isread from one half of the buffer and written to the other half of thebuffer. The data that was just written to the buffer is then shifted tothe half of the buffer where data is read from.

The Recording Parameters screen enables the user to define the rate ofthe ADC 18, the rate of the DAC 16 (which must be a multiple of the A/Drate) and the epoch duration (i.e. the size of the input buffercontained in the ADC 18). The user may also define an artifact rejectionlevel, calibration coefficients, phase adjustment coefficients andwhether on-line computations are made upon weighted or un-weighted (i.e.raw) data. The user may also choose amplification values for dataacquisition boards which provide amplification such as the AT-MIO-16e-10board. The artifact rejection level may be based on an absolutethreshold value or upon the average amplitude of the high frequencyrange of the sensed EEG data.

The View Stimulus screen enables the user to view the SSAEP stimuli thatwill be presented to the subject 60. The View Stimulus screen alsoallows the user to view the amplitude spectra of the SSAEP stimuli.

The Record Data screen allows the user to view the sensed EEG data forthe current epoch that is being sampled. The user can also view theamplitude spectra of the average sweep (a sweep is a concatenation ofepochs and the average sweep is the result from averaging a plurality ofsweeps). When the average sweep is displayed, the frequencies of theSSAEP responses in the EEG data are highlighted for easy comparison withbackground EEG activity (i.e. background noise). The Record Data screenalso allows the user to control the acquisition of the EEG data. Inaddition, the Record Data screen allows the user to view both thenumerical and graphical results of statistical analyses that areconducted on the EEG data to detect the presence of at least one SSAEPresponse to the SSAEP stimulus.

The Process Data screen enables the user to choose different methods ofviewing, storing, combining and analyzing data sets, which eithercontain sweeps or SSAEP responses from a single subject or from aplurality of subjects. The data sets may be combined so that each sweepthat goes into a final average is weighted by the amount of data fromwhich it is created or by the number of separate data sets combined. Thedata sets may also be subtracted, in order to enable the user tocalculate, for example, derived-band responses.

The software program 40 has options for collecting and displaying data.For example, as is commonly incorporated into clinical audiometricdevices, the parameters for several clinical protocols can be stored inseveral parameter files to enable several tests to be run automatically,for example, each with different stimulus intensities or different SSAEPstimuli. The results for tests incorporating different SSAEP stimuli anddifferent stimulus intensity levels can be displayed in several TestSummary screens where all of the audiometric test results of the subject60 are presented, for example, in traditional audiogram format.

In another embodiment of the objective audiometric test apparatus 10,the hearing tests may be performed partly automatically or fullyautomatically and may be used to adjust a hearing aid. In the case of ahearing aid, the gain of the hearing aid may be adjusted by theobjective audiometric test apparatus 10 according to the outcome ofaided hearing tests that are conducted. For example, during thecalibration of the hearing aid, the gain of the hearing aid for aspecific frequency region may be automatically increased if an SSAEPresponse to a given SSAEP stimulus in that specific frequency region wasnot detected. In this embodiment, the objective audiometric testapparatus 10 can communicate with the hearing aid device using aphysical connection, such as a ribbon cable, or via RF telemetry as isused to adjust other biomedical devices (such as implanted stimulators).

FIG. 1b illustrates the general steps undertaken by the objectiveaudiometric test apparatus 10. The objective audiometric test apparatus10 first generates a test signal in step M1 which is appropriate intesting an aspect of the auditory system of the subject 60. The testsignal comprises a wide variety of signals including tones, noise,amplitude modulated signals, frequency modulated signals, optimum vectoramplitude and frequency modulated signals, independent amplitude andfrequency modulated signals, signals which have envelopes that aremodulated by an exponential modulation signal, and the like.Accordingly, this step may comprise selecting a test signal and thenmodulating the test signal to obtain a modulated test signal. Thisprocedure may also be done on more than one test signal so that the testsignal comprises at least one modulated test signal. The next step M2 isto transduce the test signal to create a stimulus and present thisstimulus to the subject 60. The next step M3 is to record the EEG dataof the subject 60 simultaneously with presentation of the stimulus. Thepresentation of the stimulus and the acquisition of the EEG data must besynchronized with the objective audiometric test apparatus 10 toaccurately represent signals of interest. The next step M4 consists ofanalyzing the recorded EEG data to determine whether there are anyresponses present in the EEG data. This step will typically involveperforming a noise reduction method on the EEG data and then applying adetection method to the noise reduced data. The next step M5 may be toreport test results. The steps outlined in FIG. 1b may be part of alarger audiometric test that will involve performing each of the stepsseveral times. These particular audiometric tests and the steps whichare involved are discussed in more detail below.

SSAEP Detection

The EEG data that is sensed during the presentation of a multiple SSAEPstimulus contain superimposed responses to the multiple components ofthe SSAEP stimulus as well as background noise. Accordingly, it isdifficult to distinguish the SSAEP responses in the time domain.However, if the EEG data is converted into the frequency domain, using aFast Fourier Transform (FFT) for example, the amplitude and phase ofeach SSAEP response can be measured at the specific frequency of eachmodulation signal in the multiple SSAEP stimulus.

As previously stated, the SNR of the SSAEP response is very small.Accordingly, a large amount of EEG data needs to be collected toincrease the SNR of the SSAEP data. Conventional approaches to increasethe SNR of the SSAEP response include artifact rejection and timeaveraging. These conventional approaches are implemented by the analysismodule 46 since these techniques are still fairly popular withclinicians and research scientists in the field of audiometry.

As previously mentioned, epochs of EEG data are acquired during SSAEPtesting. Artifacts may contaminate the data and introduce large noisespikes that are due to non-cerebral potentials such as movement offacial muscles or the like. Accordingly, artifact rejection involvesanalyzing each epoch to determine if the epoch contains data points thatare higher than a threshold level such as 80 μV. Artifact rejection isuseful in removing spurious noise components to noise reductiontechniques such as time averaging to be more effective. The noisereduction module 48 is adapted to effect artifact rejection on theepochs which are recorded. If an epoch is rejected, the next epoch thatdoes not exceed the artifact rejection threshold is concatenated to thelast acceptable epoch. This concatenation procedure does not causediscontinuities in the data because the SSAEP stimulus which evokes theSSAEP response is constructed so that each epoch contains an integernumber of periods of the SSAEP response.

Time averaging comprises concatenating epochs to form sweeps. Aplurality of sweeps are then averaged in time to yield an average sweep.Time averaging reduces the level of background noise activity that arenot time-locked to the stimuli. After the average sweep is obtained, itis converted into the frequency domain via the FFT. In this case, thesweep duration is an issue since increasing the sweep durationdistributes the background noise power across more FFT bins withoutaffecting the amplitude of the SSAEP response which is confined to asingle FFT bin since the SSAEP response occurs at a single frequency andthe noise is broadband. Thus increasing the duration of the sweepincreases the frequency-resolution of the FFT. The specific frequenciesavailable from the FFT are integer multiples of the resolution of theFFT which is 1/(Nt), where N is the number of data points and t is thesampling rate. One possible implementation uses a sampling rate of 1000Hz, an epoch length of 1024 points and sweeps that are 16 epochs long(16,384 points). Accordingly, the resulting frequency resolution is 0.61Hz (1/(16*1.024*0.001)) and the frequency region in the FFT spans DC (0Hz) to 500 Hz. Alternatively, sweeps may also be 8 epochs long or 12epochs long.

The detection module 50 may provide a noise estimate which is derivedfrom neighboring frequencies in the amplitude spectrum (i.e. FFT) atwhich no SSAEP response occurs. If there were no SSAEP response in therecorded data then the power at the modulation frequency, where theresponse should occur, would be within the range of the noise power atthe neighboring frequencies. An F-ratio may then be used to estimate theprobability that the amplitude at the modulation frequency in theresulting FFT is not statistically different from the noise estimate.When this probability is less than 0.05 (p<0.05), the SSAEP response maybe considered significantly different from noise, and the subject 60 isconsidered to have heard the SSAEP stimulus. A more stringent criteriaof p<0.01 can also be chosen. Currently, the objective audiometric testapparatus 10 provides an F-Ratio where each SSAEP response in theamplitude spectrum associated with a frequency of modulation is comparedto the FFT data in 60 noise bins above and 60 noise bins below the FFTbin that contains the SSAEP response. Accordingly, this ratio isevaluated as an F-statistic with 2 and 240 degrees of freedom.

The objective audiometric test apparatus 10 further comprises the noisereduction module 48 which may be adapted to employ artifact rejection inwhich epochs are rejected based on high frequency activity. Artifactrejection which simply chooses a threshold based on the sensedpotentials may not be optimally efficient because low-frequencyhigh-amplitude EEG activity (e.g. less than 20 Hz) dominates theamplitude of the sensed EEG data. Thus the noise in the vicinity of theSSAEP response, which may be in the frequency range of 70 to 200 Hz, isnot appropriately represented by the recorded EEG data. Accordingly,rejecting epochs based upon the mean amplitude of the high frequency EEGnoise may be more appropriate.

In addition, the noise reduction module 48 may employ an adaptiveartifact rejection method in which an adaptive threshold value iscalculated which depends on a statistical property of the data points inan epoch. This method comprises calculating the standard deviation ofthe data points in the epoch and setting the threshold value to be twotimes the calculated standard deviation value. If the epoch containsdata which is over this threshold limit, then the epoch is rejected.This rejection method may be performed offline, after the sensed EEGdata has been recorded from the subject 60 or the method may beperformed online while the sensed EEG data is being recorded from thesubject 60. The online artifact rejection method may provide an idea ofhow much EEG data needs to be recorded from the subject 60 based on thenumber of epochs which are rejected. Alternatively, this adaptiveartifact rejection method can be done after the epoch is filtered by abandpass filter having a passband which is substantially similar to thefrequency region in which the SSAEP response may occur (i.e. 70 to 110Hz or the frequency range of 120 to 250 Hz). Alternatively, otherstatistical measures may be used to adaptively set the artifactrejection threshold.

The noise reduction module 48 may further employ sample weightedaveraging to reduce the noise in the sensed EEG data. Under the sampleweighted averaging method, epochs are concatenated into sweeps ofsufficient duration. A plurality of sweeps are formed and aligned suchthat a matrix is formed where the sweeps are the rows of the matrix andthe epochs are the columns of the matrix. Each sweep is filtered and theepochs along each column are then weighted by an estimate of the noisevariance that is local to the frequency region in which the SSAEPresponse resides. The noise variance estimate is local to the frequencyregion in which the SSAEP response resides because the bandpassfiltering of the sweeps is done such that the passband of the filter issubstantially similar to the frequency region in which the SSAEPresponse should occur. For example, the passband of the bandpass filtermay be from 70 to 110 Hz. The epochs are then weighted by the inverse ofthis noise variance after the noise variance has been normalized. Theweighted epochs along each column of the matrix is then summed to yielda resulting sweep which has a reduced noise component compared to thecase of simply performing time averaging on the plurality of sweeps.Alternatively, noise weighted averaging may be used where the amplitudesof the steady-state responses are removed from the noise estimate.

In pseudo-code format, sample weighted averaging is effected accordingto the steps of:

a) obtaining a plurality of epochs while sensing EEG data from thesubject 60 while simultaneously presenting the SSAEP stimulus;

b) forming a plurality of sweeps by concatenating the epochs together;

c) filtering each sweep to obtain a plurality of filtered sweeps;

d) aligning each sweep to form a first matrix in which the sweeps arethe rows of the matrix and the epochs within the plurality of sweeps arethe columns of the matrix and aligning each filtered sweep in a similarfashion to form a filtered matrix which is used to calculate weights;

e) calculating the variance of each epoch in the filtered matrix toobtain a noise variance estimate for each epoch in the filtered matrix;

f) normalizing the noise variance estimate for each epoch in thefiltered matrix by dividing the noise variance estimate for each epochin the filtered matrix by the sum of all noise variance estimates forthe epochs along the column of the filtered matrix which contains theepoch to obtain a normalized noise variance estimate for each epoch;

g) inverting each normalized noise variance estimate to obtain a weightfor each epoch and multiplying each corresponding epoch in the firstmatrix by its respective weight to obtain a plurality of weightedepochs; and,

h) summing all of the weighted epochs in the first matrix along thecolumns of the first matrix to obtain a signal estimate.

Referring to FIGS. 2a-2 h, sample weighted averaging and artifactrejection (based on the mean amplitudes of higher frequency regions) arecompared to normal averaging. The results show that sample weightedaveraging results in SSAEP responses with the higher SNR. FIG. 2a showsa histogram for amplitudes of recorded EEG data points for a noisyrecording (i.e. there were many artifacts during the recording, theamplitudes of which are identified by arrows in FIG. 2a). FIG. 2b alsoshows a histogram for amplitudes of the recorded EEG data for a quietrecording (i.e. there were not many artifacts during the recording). Thedata points in FIGS. 2a and 2 b were obtained from the same subject whowas presented a multiple SSAEP stimulus comprising eight test signals at50 dB SPL. FIGS. 2c and 2 d show the results from analyzing the datausing normal averaging. The SSAEP responses that have been detected aredenoted by the filled arrowheads. In FIG. 2c, only four of the eightSSAEP responses have been detected meanwhile in FIG. 2d, all eight ofthe SSAEP responses have been detected. FIGS. 2e and 2 f show theresults from analyzing the EEG data using sample weighted averaging. InFIG. 2e, seven SSAEP responses have been detected and in FIG. 2f, eightSSAEP responses have been detected. Furthermore, comparing FIG. 2e withFIG. 2c shows that sample weighted averaging has detected 3 more SSAEPresponses as well as increased the average SNR of the SSAEP responses bya factor of over 2. FIGS. 2g and 2 h show the results from analyzing theEEG data using amplitude rejection in which rejection was based on themean amplitude of the EEG data in the higher frequency region. FIG. 2gshows that this form of artifact rejection resulted in seven SSAEPresponses being detected while FIG. 2h shows that all eight SSAEPresponses were detected.

Referring now to the detection module 50, a phase weighted t-test isused to detect the presence of SSAEP responses in the recorded EEG data.The phase weighted t-test employs data biasing to detect the SSAEPresponse based on a priori knowledge about the SSAEP response. Inparticular, if the phase of the SSAEP response is known, then the EEGdata can be biased so that statistical analysis (i.e. the detectionmethod) is more likely to recognize an SSAEP response with a phase thatis similar to the expected phase than noise data with a completelydifferent phase. The biasing of the data points is done by employing aweighting function that provides larger weights for SSAEP responses thathave a phase which is close to the expected phase value. The phaseweighted t-test allows phase-weighting without the need for empiricalcompensation of the probability level at which the SSAEP response isdetected.

Since the recorded EEG data is processed by the FFT, the resulting datapoints are two-dimensional and have real and imaginary components. TheFFT bins which represent the SSAEP response and the surrounding noisecan be projected onto a single dimension oriented at the expected phaseby using the equation:

p _(i) =a _(i)*cos(θ_(i)−θ_(e))  (1)

where

p_(i) is the projected value;

a_(i) is the amplitude of an FFT component (i.e bin);

θ_(i) is the phase of the FFT component; and,

θ_(e) is the expected phase of the response.

An upper confidence limit, based on the amplitude of the projected FFTcomponents that contain noise can then be estimated using a one-tailedStudent t-test with p<0.05 (to reduce the number of false positives,p<0.01 can be used). An SSAEP response can then be recognized as beingstatistically significantly greater than noise (i.e. detected) if theprojected value of the FFT component whose frequency is the same as thatof the SSAEP responses which is being detected is larger than the upperconfidence limit.

The steps to employ the phase weighted t-test on the EEG data pointsinclude the following steps:

a) forming a plurality of sweeps from the EEG data points;

b) averaging the plurality of sweeps to obtain a plurality of averageddata points;

c) calculating a plurality of Fourier components for the plurality ofaveraged data points wherein the Fourier components are calculated forthe frequency region where the response should occur and adjacentfrequencies thereof (for noise estimation);

d) calculating the amplitude (a_(i)) and phase (θ_(i)) for the pluralityof Fourier components which were calculated in step (c);

e) biasing the amplitudes (a_(i)) to obtain biased data points (p_(i))according to the formula:

p _(i) =a _(i)*cos(θ_(I)−θ_(e))

where θ_(e) is the expected phase value (this is done for the Fouriercomponent at which the response should occur as well as for adjacentFourier components which represent noise);

f) calculating upper confidence limits using a one tailed Student t-teston the biased amplitudes which represent noise in the vicinity ofFourier components where the response should occur; and

g) comparing biased amplitudes of Fourier components where the responseshould occur to the upper confidence limits to determine if the biasedamplitudes are larger than the upper confidence limit.

If the biased amplitudes for the response is larger than the upperconfidence limit, then the response is detected; otherwise no responseis detected. Note that in the above method, some preprocessingtechniques can be used on the plurality of data points to reduce thebackground EEG noise amplitude in the plurality of data points by thenoise reduction module 48. These preprocessing techniques may includeartifact rejection, adaptive artifact rejection, time averaging andsample weighted averaging.

Referring to FIGS. 3a and 3 b, the use of a phase weighted t-test isillustrated on some sample data. FIG. 3a shows that the response 70 iswithin the upper confidence limits which are defined by the circle 72.The upper confidence limits 72 were obtained according to the procedurepreviously described using two-dimensional F-statistics. The SSAEPresponse 70 is therefore not statistically significant (i.e. notdetected) since the magnitude of the SSAEP response 70 is not largerthan the upper confidence limit 72. However, knowing that the expectedphase of the SSAEP response should be 104 degrees, in this example,allows for the use of the phase weighted t-test which is shown in FIG.3b. Biasing the FFT components that represent noise results (i.e. theopen circles in FIG. 3b) results in upper confidence limits 76 that arenow shown by a parabola (the actual upper confidence limit is the singlepoint where the parabola intersects the line of the expected phase).Notice that the apex of the parabola 76 has a smaller excursion from theorigin as compared to the circle 72 which makes it easier to detect anSSAEP response if its phase is similar to that which is expected. TheSSAEP response is now biased by projecting it on the expected phase of104 degrees. The SSAEP response 74 now extends beyond the confidencelimits 76 of the projected noise measurements. The SSAEP response isconsidered to be statistically significant and therefore detected withthe same number of noise data points shown in FIG. 3a.

In the case of the multiple SSAEP stimulus which contains multiplesignals that evoke multiple SSAEP responses, the phase weighted t-testis repeated for each of the expected SSAEP responses, with each responsehaving a different expected phase since they were evoked by test signalshaving various carrier frequencies.

The formula in equation (1) for biasing the amplitudes based on thedifference between the measured phase and the expected phase can be madebroader or tighter depending on the weighting function that is used. Forinstance, the cosine weighting in equation 1 can be replaced with acosine squared function in order to more strongly punish values thatdeviate from the expected phase. Alternatively, the ‘tightness’ of theweight function can be adjusted according to the normative inter-subjector intra-subject variance of expected phase values. For example, thestandard deviation for the measured phase values in a normativedatabase, contained within the master database 52 can be normalized andused to weight the difference between the expected and observed phasesin equation 1.

Furthermore, phase coherence measurements can be biased toward anexpected phase in exactly the same way as was done for the phaseweighted t-test, whereby the phase coherence estimate is biased by thedifference between the expected and observed phases. Additionally, thesemethods of biasing the data can be used to evaluate responses in averagesweeps, single sweeps, or even across individual epochs.

Alternatively, the average sweep is not always calculated and thepresence of a response can be statistically assessed by assessing theSSAEP responses for each sweep or even for each epoch.

Several approaches can be used to define the expected phase. First andforemost, a database of normative expected phase values are collectedand stored in the master database 52. These normative expected phasevalues can be obtained by collecting the average phases of SSAEPresponses obtained from a group of normal subjects (who may be matchedfor age and gender) who were presented with similar stimuli. Anotherapproach is to estimate the expected phase from previously recorded dataon the particular subject who is currently being tested. For example, ifan SSAEP response at 60 dB SPL has a phase of 80 degrees then one mayuse this value or a slightly smaller phase as the expected phase for theSSAEP response that is recorded during a test with a similar SSAEPstimulus that is at 50 dB SPL. Alternatively, the phase measured fromseveral earlier sweeps of a recording period may be used to obtain anestimate of the phase for the sweeps which are taken later in therecording period. Another alternative would be in the case of themultiple SSAEP stimulus which contains individual SSAEP stimuli atdifferent carrier frequencies. In this case, the phases of the SSAEPresponses that have reached statistical significance (i.e. beendetected) during a given testing period can be used to estimate what thephase should be for SSAEP responses that have not yet reachedsignificance. For example, if the phase of a response to an SSAEPstimulus with a carrier signal comprising a 1000 Hz tone that isamplitude modulated at 80 Hz is 45 degrees and the phase of a responseto an SSAEP stimulus with a carrier signal comprising a 4000 Hz tonethat is amplitude modulated at 90 Hz is 90 degrees then the predictedphase for an SSAEP response to an SSAEP stimulus that with a carriersignal comprising a 2000 Hz tone that is amplitude modulated at 85 Hzthat has not reached significance may be interpolated as being 60degrees. Other interpolation methods may be used.

The detection module 50 may be further adapted to perform a phase zonemethod to detect the presence of SSAEP responses in recorded EEG data.While the statistical detection methods of the prior art, rely on theprobability of phases randomly occurring, a statistical detection methodmay be made stronger if the distribution of phases is expected to liewithin a given number of degrees (i.e. the target phase range) from anexpected phase value. For example, if the expected phase of a givenresponse is 90 degrees, then the chance of the phase value landingwithin N degrees of the expected phase is (N/360). Accordingly, if N isset at 90 and the expected phase is 70 degrees, then there is a 1 in 4chance that each calculated phase value of a recording without an SSAEPresponse will occur between 25 and 115 degrees (i.e. the target phaserange). The number of phases that fall within the target phase range canbe compared to the number of phases which fall outside of the targetphase range using binomial analysis, with a probability of 0.25 (i.e., 1in 4) for this example. However, if the variance of the calculatedphases is small, then the target phase range may also be made smaller(i.e. a target phase range less than 90 degrees). This will allow theSSAEP response to become statistically significant (i.e. detected) witha smaller amount of EEG data points since the binomial probability indexwill get smaller as the target phase range gets smaller

In terms of SSAEPs, the phase zone method is effected by analyzing eachsweep of data rather than using the average sweep as is done in theF-test. The method consists of the steps of:

a) presenting the SSAEP stimulus to the subject 60;

b) sensing the EEG from the subject 60 while substantiallysimultaneously presenting the stimulus to the subject 60 to obtain aplurality of data points;

c) separating the plurality of data points into sweeps;

d) calculating a Fourier component for the frequency at which theresponse should occur for each sweep;

e) calculating a phase value for each Fourier component;

f) calculating a phase target range;

g) calculating the number of phases (Na) from step (e) that are withinthe target phase range; and,

h) using binomial analysis to analyze Na to determine whether saidplurality of data points contains a response.

The phase target range can be calculated based on a database ofnormative expected phases, stored within the master database 52, thatare correlated to the subject and the SSAEP stimulus characteristics.

The detection module 50 may be further adapted to perform anotherstatistical method for detection referred to as the MRC method. The useof an expected phase angle has been incorporated as a variant of theRayleigh test for circular uniformity (RC) termed the modified Rayleightest (MRC). The RC method can be made more statistically powerful if anexpected phase angle is known. Hence the MRC is developed which weightsthe RC value with a weighting function that incorporates the expectedphase according to the following equation:

MRC=RC*cos(θ_(a) −θ _(e))  (2)

where θ_(a) is the vector-averaged angle of the data set and θ_(e) isthe expected angle.

In addition to detecting responses at the modulation frequencies used inthe SSAEP stimulus, the detection module 50 may be adapted to detect theSSAEP responses that occur at the carrier frequency. In this case, thesampling rate of the ADC 18 would have to be increased so that EEG datawith frequency content in the range of the carrier frequency could beproperly sampled. However, this EEG data may be difficult to interpretbecause stimulus artifacts, which are created by electromagneticinductance, will add to the sensed EEG data and distort the data. Theseartifacts can be minimized by various techniques such as shielding thetransducer 26 and separating it from the recording instrumentation (i.e.the sensors 28, the amplifier 30 and the filter 32). In addition, theuse of insert earphones with a greatly extended air-tube can aid inovercoming stimulus artifacts (provided that the transfer function ofthe transducer is adapted to compensate for the filtering effect of thelengthened tube). Another technique to remove the stimulus artifact maybe based on the fact that the stimulus artifact changes its amplitudelinearly with changes in the intensity of the SSAEP stimulus while thelatency of the stimulus artifact does not change, whereas an SSAEPresponse will show non-linearities in its intensity relationship andwill change latency. Thus, if EEG data is recorded at more than onestimulus intensity, algorithms may be constructed to remove the stimulusartifacts based on its linearity with respect to stimulus intensity.

SSAEP Stimuli

The objective audiometric test apparatus 10, via signal creator 42 isadapted to construct a variety of test signals which can be used in theSSAEP stimulus. These test signals include tones, amplitude modulatedsignals (AM), frequency modulated signals (FM), an optimum vectorcombined amplitude modulated and frequency modulated signals (optimumvector mixed modulation or OVMM) and independent amplitude modulated andfrequency modulated signals (IAFM). The modulator 44 may also be usedwith the signal creator 42 to provide envelope modulation with anexponential modulation signal. The signal creator 42 can also generatetest signals consisting of broad-band noise, high-pass noise, low-passnoise, or band-pass noise all of which can be either modulated orunmodulated.

The signal creator 42 generates the optimum vector combined amplitudemodulation and frequency modulation (OVMM) test signal such that theamplitude modulation rate is the same as the frequency modulation rate.Furthermore, the phase of the frequency-modulated component of the OVMMtest signal is adjusted with respect to the phase of the amplitudemodulated component of the OVMM test signal such that the SSAEP responseevoked from the subject has an increased amplitude.

Referring to FIGS. 4a-4 f, the amplitudes of SSAEP responses to an SSAEPstimulus consisting of an AM test signal (open square data points) andan SSAEP stimulus consisting of an MM test signal (filled in circulardata points). The data was collected from eight test subjects. The SSAEPresponses were obtained for a variety of carrier frequencies, stimulusintensities and frequency modulation depths. FIGS. 4a-4 f show that theMM SSAEP stimulus evoked SSAEP responses with larger amplitudes than theAM SSAEP stimulus for a variety of stimulus intensities and carrierfrequencies. The frequency modulation depth for the MM SSAEP stimuluswas 25% for FIGS. 4a-4 c and 10% for FIGS. 4d-4 f (the frequencymodulation depth indicates the frequency deviation from the carrierfrequency in the SSAEP stimulus). The amplitude modulation depth was100% for the AM SSAEP test results shown all Figures. In FIGS. 4a-4 c,at 50, 40 and 30 dB SPL, the amplitudes of the SSAEP response were 30%,49%, and 28% larger for responses evoked by the MM SSAEP stimulus ascompared to those evoked by the AM SSAEP stimulus. FIGS. 4d-4 f showresults from a different group of eight subjects in which the SSAEPresponse amplitudes were 20%, 7%, and 8% larger when using MM SSAEPstimuli. These Figures also show that it is possible to obtain enhancedresponse amplitudes near threshold using frequency modulation depthsnear 25%.

For the test results shown in FIGS. 4a-4 h, the MM SSAEP stimuli werenot adjusted to evoke SSAEP responses with optimal amplitudes. When anoptimum vector mixed modulation signal is used, the SSAEP responseamplitudes are larger for the higher frequency test results (i.e. 4000to 6000 Hz). It should also be noted that the use of an FM depth of 10%in the MM SSAEP stimulus is not sufficient to increase the SSAEPresponse amplitudes as compared to the response amplitudes obtained whenusing the AM SSAEP stimulus. Rather, an FM depth of 25% is needed toevoke larger amplitude SSAEP responses as is shown in FIGS. 4b and 4 c.

Referring to FIG. 5a, adjusting the phase of the FM component of the MMSSAEP stimulus relative to the phase of the AM component results inSSAEP responses with different amplitudes. At one particular phase theresponse will be larger than at other phases. This is the basis of theoptimum-vector mixed modulation stimulus. This adjustment is based onthe principle that the SSAEP response 80 to an MM SSAEP stimulus is thevector sum of the response to the AM component 82 of the SSAEP stimulusand the FM component 84 of the SSAEP stimulus, in which the componentsare independent or only interact to a small degree. The SSAEP responseto the AM component 82 is shown as the vector originating at the origin.Added to the SSAEP response to the AM component 82 is the response tothe FM component 84 of the SSAEP stimulus which has a different phasethan the SSAEP response to the AM component 82. These SSAEP responseswere evoked by an SSAEP stimulus in which the FM component had the samephase as the AM component so that the relative phase between the AM andthe FM components of the SSAEP stimulus was zero. As is evident fromFIG. 5a, this results in an SSAEP response to the MM stimulus that issmaller than the response to the AM stimulus alone.

Referring to FIG. 5b, if the phase of the FM component of the SSAEPstimulus was adjusted relative to the phase of the AM component of theSSAEP stimulus in a range from 0 to 360 degrees there would be avariation in the amplitude of the response 86 which could be modeled asa sinusoid. Increases in the phase of the FM component of the SSAEPstimulus will rotate the FM response vector 84 clockwise. The SSAEPresponse to the MM SSAEP stimulus for any phase value of the FMcomponent relative to the AM component of the SSAEP stimulus can then beobtained by drawing a vector straight up from the x-axis to the responsecurve 88. This line would be drawn at the point on the x-axis which isthe value of the phase of the FM component with respect to the AMcomponent for the SSAEP stimulus. One example of a possible response isshown as the dotted-line vector 86 which also happens to be the largestresponse amplitude. In this case, the SSAEP response 84 to the FMcomponent of the SSAEP stimulus and the response 82 to the AM componentof the SSAEP stimulus will line up to produce an optimum vectormixed-modulation (OVMM) stimulus, which should produce the largest MMresponse when the relative phase between the phase of the FM componentand the AM components of the SSAEP stimulus is φ. FIG. 5b thus indicateshow the angle (p can be derived using several MM response amplitudesobtained as the relative phase between the FM and AM components of theSSAEP stimulus is varied. The resulting SSAEP response amplitudes can befit with a sine wave having a baseline-offset (as shown in FIG. 5b). Thesize of the baseline-offset is equivalent to the amplitude of responseto the AM component of the SSAEP stimulus and the amplitude of the sinewave is equivalent to the amplitude of the response to the FM componentof the SSAEP stimulus.

These stimuli are called optimum-vector mixed modulation (OVMM) stimulito distinguish then from other MM stimuli where the relative phases ofthe AM and FM components are arbitrarily set. OVMM SSAEP stimuli cantherefore be used to evoke SSAEP responses with larger amplitudes. Thisis beneficial since SSAEP responses with larger amplitudes have largerSNR which should allow for SSAEP response detection in a smaller amountof time.

The process to test with OVMM SSAEP stimulus follows the followingsteps:

a) create a test signal which contains at least one combined amplitudemodulation and frequency modulation signal in which the phase of the FMcomponent of the test signal is adjusted relative to the phase of the AMcomponent of the test signal such that an increased response can beevoked from a subject;

b) creating the test signal further comprises choosing the carrierfrequency of the AM component to be substantially similar to the carrierfrequency of the FM component and the modulation frequency of the AMcomponent to be substantially similar to the modulation frequency of theFM component;

c) transducing the test signal to create an acoustic stimulus andpresenting the acoustic stimulus to the subject;

d) sensing potentials from the subject while substantiallysimultaneously presenting the acoustic stimulus to the subject; and,

e) analyzing the sensed potentials to determine whether they containdata indicative of at least one steady-state response to the acousticstimulus.

Testing with OVMM SSAEP stimuli preferably utilizes a database ofnormative optimal phase values, stored within the master database 52,that may be used to adjust the phase of the FM component of the OVMMSSAEP stimulus relative to the AM component of the OVMM SSAEP stimulus.This basically entails creating a database of normative values. Thedatabase is stored in the master database 52. The database of normativevalues can contain phase difference data (i.e. difference between thephases of the FM and AM components of the OVMM SSAEP stimulus) which iscorrelated to subject characteristics and stimulus characteristics.Subject characteristics typically include the age of the subject, thesex of the subject and the like. Stimulus characteristics include thecarrier frequency of the FM or AM component, the intensity of thestimulus, the AM modulation depth, the FM modulation depth and the like.

The following steps may be followed to create the database of normativephase difference data:

a) Select a sample population to test which is correlated to a group ofsubjects who will be examined; i.e., if testing will be performed onnewborn infants then a sample of 100 subjects may be tested, who may beappropriately matched for age and for sex;

b) Record EEG data from the sample population that contain responses toSSAEP stimuli which contain AM components only and responses to SSAEPstimuli which contain FM components only; this testing should be donefor each of the stimuli which will be used when examining the group ofsubjects from step (a);

c) Detect the SSAEP responses in the recorded EEG data and measure thephase of each SSAEP response;

d) Calculate the differences in the phases measured in step (c) betweenthe SSAEP responses to the AM component only SSAEP stimuli and the FMcomponent only SSAEP stimuli and plot the resulting SSAEP responseamplitudes (which is the vector summation of the responses to the AMcomponent only SSAEP stimuli and the FM component only SSAEP stimuli) toobtain a waveform as shown in FIG. 5b; and,

e) Find the phase difference for which the resulting vector summation ofthe response amplitudes for the AM and FM components result in a maximumamplitude; this phase value is then used in the OVMM SSAEP stimulus toevoke increased responses from test subjects.

The signal creator 42 can also generate another test signal comprisingan AM component and an FM component wherein these two components areindependent from each other in that they evoke SSAEP responses that areindependent from each other. This property holds true for a multipleSSAEP stimulus that contains multiple independent AM and FM components.Accordingly, the test signal is referred to as an independent amplitudeand frequency modulation signal (IAFM). The IAFM signal has an AMmodulation frequency that is different from the FM modulation frequency.

Referring to FIG. 6a, a partial view of the amplitude spectrum of theSSAEP responses for a multiple SSAEP stimulus that contains AM testsignals having 100% amplitude modulation depth is shown in the toppanel. The middle panel shows a partial view of the amplitude spectrumof the SSAEP responses for a multiple SSAEP stimulus that contains FMtest signals having 20% frequency modulation depth and the bottom panelshows the SSAEP responses for a multiple SSAEP stimulus that containsIAFM test signals. The frequencies of the SSAEP responses are indicatedby inverted triangles. FIG. 6b shows the corresponding polar plots foreach of the SSAEP responses. The circles represent the confidence limitsof each SSAEP response. If the circle does not contain the origin, thenthe SSAEP response can be considered to be statistically significantly(p<0.05) different from the background noise and therefore detected. TheSSAEP responses in the bottom panel tend to be slightly smaller inamplitude than those obtained when only one type of modulation was usedin the SSAEP stimulus. However, the phases between the SSAEP responsesin the bottom panel and the corresponding responses in the top andmiddle panels are quite similar. Thus, IAFM stimuli allow for theindependent testing of the parts of the auditory system that respond toamplitude modulation and the part of the auditory system that respondsto frequency modulation. The separate SSAEP responses (i.e. in responseto either an AM or FM SSAEP stimulus) can be used to evaluate hearingfor a particular frequency region by incorporating both of these SSAEPresponses to evaluate if a SSAEP response is present using the Stouffermethod.

Experimental results have also shown that FM SSAEP stimuli can evokeSSAEP responses when the frequency modulation depths as little as 2% areused. Furthermore, SSAEP responses can be evoked when using faster FMmodulation rates. Experimental data has also shown that SSAEP responsesto FM stimuli could be elicited at these rapid rates and low modulationdepths while producing a different phase as compared to the SSAEPresponse to an AM SSAEP stimulus. This suggests that the FM SSAEPstimulus was being processed differently than the AM SSAEP stimulus.This is important for testing paradigms that will rely on these fasterrates. Furthermore, experimental data has shown that the use of AM andFM stimuli presented at supra-threshold intensities with depths ofmodulation less than 100% and rates of modulation over 70 Hz evoke SSAEPresponses whose amplitudes have a correspondence to behavioralthresholds.

The signal creator 42 is also adapted to create a test signal comprisinga carrier signal that is modulated by two modulation signals thatmodulate in the same manner but at different modulation rates. This testsignal may be called a dual modulation signal. For instance, there maybe 2 modulation signals which amplitude modulate a carrier signal atdifferent modulation rates. An SSAEP stimulus based on this type of testsignal may be useful in certain situations. For instance, the two SSAEPresponses that are evoked by the dual modulation SSAEP stimulus can beevaluated using the Stouffer Method. Additionally, it may be useful toset the lower modulation rate in the 30-40 Hz range while the highermodulation rate is in the 70-90 Hz range. Accordingly, by using theStouffer method, responses for both the 30-40 Hz and 70-90 Hz ranges canbe simultaneously assessed. If a subject is alert, the SSAEP responsesto lower modulation rates will become significant faster, while asubject that begins to doze off may cause the SSAEP responses to thefaster stimuli to become significant faster.

The Stouffer method statistically compensates for trying to detect 2rather than 1 response, however, a potential drawback may occur when anSSAEP response that is highly significant when evaluated independentlyfails to reach significance when assessed in concert with another SSAEPresponse. For example, an 80 Hz SSAEP response that is highlysignificant should not be assessed as being not significant if it iscombined with a 40 Hz SSAEP response that is far from being significant.The software program 40 can compensate for this by using Bonferronicorrections or by allowing the user to choose different criteria fordetecting an SSAEP response (i.e. choosing the p<0.01 or p<0.001criteria instead of the p<0.05 criteria).

One possible use of the dual modulation test signal may be to monitorthe level of arousal for a patient who is subjected to anesthesia. Theuse of anesthesia will reduce the amplitude of the 40 Hz SSAEP responsebut not the amplitudes of the higher frequency SSAEP responses. Thus, toensure that the measurement of data is not contaminated by someperipheral dysfunction, such as the earphone not working correctly orthe ear developing a conductive hearing loss, it would be beneficial tomonitor both the 40 Hz and 80 Hz SSAEP responses simultaneously. The 80Hz SSAEP response could be used to demonstrate that the peripheralauditory function of the subject 60 patient is normal. Another use forthe dual modulation test signal is in the assessment of the ability ofthe subject 60 to process temporal modulation functions (as is discussedin greater detail below).

The signal creator 34 creates the various test signals which are used inthe SSAEP stimulus according to Equation 2. Accordingly, Equation 2 canbe used to create AM, FM, MM, OVMM and IAFM test signals.

s(i)=a[1+m _(a) sin(2πf _(am) ti)]sin(2πf _(c) ti+F(i))/(1+m _(a)²/2)^(½)  (2)

where:

F(i)=(m _(f) f _(c)/(2f _(fm)))sin(2πf _(fm) ti+θπ/180)  (3)

i is an address in the DAC output buffer

t is the DAC rate;

θ is the phase difference in degrees between the AM and FM components ofthe test signal s(t);

f_(am) is the modulation frequency for amplitude modulation; and,

f_(fm) is the modulation frequency for frequency modulation.

The test signal s(t), consists of sinusoidal tones having a carrierfrequency of f_(c). The AM is performed by the terms within the squarebrackets. The AM test signals are created by modulating the amplitude(a) of the carrier signal. The amplitude modulation depth is (m_(a))controls the influence of the modulation signal on the envelope of thecarrier signal.

The FM test signal is formed by modulating the phase of the carrierwaveform according to the function F(i) shown in equation 3. Thefrequency modulation depth (m_(f)) is defined as the ratio of thedifference between the maximum and minimum frequencies in the frequencymodulated signal compared to the carrier frequency. For example, when a1000 Hz carrier tone is frequency modulated with a depth of 25%, thefrequency varies from 875 Hz to 1125 Hz which is a deviation ±12.5% fromthe carrier frequency of 1000 Hz. The term m_(f)f_(c)/(2f_(fm))represents the frequency modulation index (often denoted by β). Thefinal divisor in equation 2 is used to maintain a constantroot-mean-square amplitude for various amounts of amplitude-modulation.

If m_(f) equals zero, then the test signal s(i) becomes an AM sinusoid.If m_(a) equals zero, then the test signal s(i) becomes an FM sinusoid.If f_(am) and f_(fm) are equal and if both m_(a) and m_(f) are greaterthan zero, then the test signal s(i) becomes an MM test signal. Iff_(am) and f_(fm) are not equal and if both m_(a) and m_(f) are greaterthan zero, the test signal s(i) becomes an IAFM test signal.

The signal creator 42 can also create a test signal in which theenvelope of the signal is modulated by an exponential modulation signal.In order to use exponential amplitude modulation, the formula for AM (inthe square brackets) of Equation 3 becomes:

└2m _(a)((((1+sin(2πƒ _(m) ti))/2)^(eam)−0.5)+1)┘  (4)

where eam is the exponent. In this equation, the test signal is adjustedto maintain the same root-mean-square value for the intensity of theresulting SSAEP stimulus regardless of the exponent used in theexponential modulation signal. In order to form an FM test signal withan exponential envelope, a running integral of the envelope equationmust be maintained. The running integral sums all the envelope values upto the present address in the buffer according to: $\begin{matrix}{_{i} = {\sum\limits_{1}^{i}x_{i}}} & (5)\end{matrix}$

and the envelope that is being integrated is:

x _(i)=(2πm _(f) f _(c) t)(((1+sin(2πf _(fm) ti))/2)^(efm)−0.5)  (6)

where efm is the exponent for the exponential modulation signal. Theintegrated value is then inserted into Equation 3 instead of the valueF(i). In addition, changes in the phase of the envelope may be made byshifting the function in time.

Referring to FIGS. 7a-7 d, the usage of exponential envelope modulationfor both AM and FM SSAEP stimuli produce larger responses than AM SSAEPstimuli. FIGS. 7a and 7 b show the percent increase in responseamplitude when using AM SSAEP stimuli with exponential envelopescompared to AM SSAEP stimuli without exponential envelopes for a 50 dBpSPL and 30 dB pSPL stimulus intensity. FIGS. 7c and 7 d show responseamplitudes in nV corresponding to the data shown in FIGS. 7a and 7 b. At50 dB pSPL the exponential envelope modulation increases the SSAEPresponse amplitude in the lower and higher frequency ranges. At 35 dBpSPL, the exponential envelope modulation increases the SSAEP responseamplitude especially for the lower frequencies.

An informal analysis of the results shown in FIGS. 7a-7 d indicates thatusing a sinusoidal signal to the power of 2 or 3 at stimulus intensitiesof 30 and 50 dB pSPL provides the larger SSAEP response amplitudes.Alternatively, fractional exponents may also be used. It should be notedthat an increase in SSAEP response amplitude by 40% will enable the testtime to be reduced by a factor of 2 when detecting the SSAEP responsesince the EEG noise decreases with the square root of the data (i.e.1.4142=(2)^(½)). Thus, modulating the envelope of the SSAEP stimuluswith an exponential signal may result in a reduction of test time.

Exponential envelopes may also be created using AM depths below 100%,such as 80%, in order to more closely maintain the steady-state natureof the SSAEP stimulus. Additionally, the use of exponential envelopestends to increase the amplitude of the SSAEP responses at harmonic ofthe modulation frequency. Accordingly, hearing tests may be utilizedwhere envelopes are modulated by an exponential sinusoid in the 40 Hzrange and the SSAEP responses are evaluated at the second harmonics ofthe modulation frequency. A detection method may also be devised inwhich the sensed EEG data is evaluated at both the modulation frequencyand the second harmonic of the modulation frequency using the Stouffermethod or a Stouffer method which defaults to the evaluation of a singleharmonic when one of the two harmonics meets some criteria (i.e. p<0.01or p<0.001).

Audiometric Testing Using Steady-state Evoked Potentials

The objective audiometric test apparatus 10 is also adapted to performvarious audiometric tests in an objective manner using SSAEP stimuliwithout any necessary user control other than the selection andinitiation of a particular audiometric test. Testing is objective in thesense that the subject does not have to subjectively respond to thestimuli used in the test and the individual conducting the test does nothave to subjectively interpret the recorded data since statisticalmethods are used to analyze the recorded data. Thus, the “completelyobjective auditory testing system” (i.e. COATS) performs these tasksobjectively. The objective audiometric test apparatus 10 may be used toevaluate hearing in subjects with normal hearing, cochlear hearing lossor abnormal auditory nervous systems, and in subjects who use hearingaids.

In general, the objective audiometric test apparatus 10 presents SSAEPstimuli, records EEG data and determines whether SSAEP responses arepresent in the EEG data. The objective audiometric test apparatus 10then presents further SSAEP stimuli to obtain more precise information.However, the individual performing the audiometric tests can makedecisions about which SSAEP stimuli to present and the duration of eachtest.

The objective audiometric test apparatus 10 is further adapted to obtainmultiple audiometric thresholds concurrently. In addition, the objectiveaudiometric test apparatus 10 is adapted to perform audiometric testingon subjects with aided and unaided hearing. With respect to aidedhearing tests, the objective audiometric test apparatus 10 can be usedto adjust the various parameters of a hearing aid. The objectiveaudiometric test apparatus 10 can also perform latency tests, AM/FMdiscrimination tests, rate sensitivity tests, aided hearing tests, depthsensitivity tests and supra-threshold tests.

The objective audiometric test apparatus 10 also utilizes one of thedatabases of normative data, stored in the master database 52, toconstruct SSAEP stimuli, detect SSAEP responses and determine whetherdetected SSAEP responses are indicative or normal or abnormal hearing.The databases contain data which is correlated by subjectcharacteristics such as age, sex and state over a variety of stimuluscharacteristics such as type of modulation, type of modulation envelope,modulation rate and modulation depth, etc. The database also containsdata about SASEP responses such as latency, the ratio of amplitudes ofSSAEP responses to AM and FM SSAEP stimuli, etc.

The objective audiometric test apparatus 10 can perform audiometrictests to assess the supra-threshold hearing of a subject. Thesupra-threshold tests comprise assessing the threshold of a subject'sauditory system in detecting changes in the frequency or intensity of astimulus at supra-threshold stimulus intensities. Accordingly, thesupra-threshold test comprises an intensity limen and a frequency limen.The intensity limen test protocol involves varying the intensity of astimulus by varying the AM depth. In particular, the intensity limeninvolves estimating the threshold for detecting a change in theamplitude modulation depth of the stimulus. The frequency limen involvesvarying the frequency content of the SSAEP stimulus by varying frequencymodulation depth. In particular, the frequency limen involves estimatingthe threshold for detecting a change in the FM depth of an SSAEPstimulus. The intensity and frequency limens correlate with thesubject's 60 ability to discriminate supra-threshold sounds of variousintensities and frequencies.

The procedure for determining the intensity limen preferably involvesthe following steps:

a) Constructing an SSAEP stimulus with an AM component having an AMdepth of 100%;

b) Recording the EEG data while presenting the SSAEP stimulus to thesubject 60;

c) Analyzing the EEG data to determine if there was a response to theSSAEP stimulus;

d) If there is a response, then decreasing the AM depth of the AMcomponent by half and repeating steps (b) and (c); and,

e) if no response is detected then the intensity limen is determined asthe lowest AM depth which resulted in an SSAEP response being detected.

Likewise, the frequency limen can be conducted according to thefollowing steps:

a) Constructing an SSAEP stimulus having an FM component with an FMmodulation depth of 40%;

b) Recording the EEG data while presenting the SSAEP stimulus to thesubject;

c) Analyzing the EEG data to determine if there was an SSAEP response tothe SSAEP stimulus;

d) If there is a response, then decreasing the FM depth of the FMcomponent by half and performing steps (b) and (c); and,

e) if no response is detected then the frequency limen is determined asthe lowest FM depth which resulted in an SSAEP response being detected.

The supra-threshold hearing test may further involve varying themodulation depths in an SSAEP stimulus and examining the size of theresulting SSAEP responses compared to population normative values.

The supra-threshold hearing test may further comprise a method whichshould be less prone to inter-subject differences. The method comprisesmeasuring the amplitude of the SSAEP response when presenting thesubject with an AM SSAEP stimulus that has an AM depth of 100%. Thisresponse amplitude may then be compared to SSAEP response amplitudesthat are obtained when presenting the subject with an AM SSAEP stimulusthat has smaller AM depths. Accordingly, a demonstrative measure may bethe ratio of the amplitude of an SSAEP response which is recorded whilepresenting a subject with an AM SSAEP having a 50% AM depth to theamplitude of an SSAEP response obtained while presenting a subject withan AM SSAEP having a 100% AM depth. As in the case of absoluteamplitudes, normative values can be obtained for these ratios usingappropriate age and sex matched control populations that were exposed tosimilar stimuli. These normative values can be obtained from one of thedatabases in the master database 52.

The objective audiometric test apparatus 10 may also perform latencytests on the subject to determine if the subject has normal or abnormalhearing. Through the use of a “preceding cycles technique”, experimentshave shown that SSAEP responses have reliable and repeatable latencyvalues in normal healthy ears. The latency values are obtained from thephases of the detected SSAEP responses. The latency of an SSAEP responseis important for diagnosing various kinds of sensorineural hearing loss.An abnormally long latency value may indicate that an acoustic neuromais present. Alternatively an abnormally short latency value may indicatethat the subject has Meniere's disorder.

Referring to FIG. 8, the results of a set of experiments using dichoticstimulation are shown. A multiple SSAEP stimulus with four carriersignals having carrier frequencies that were separated by an octave werepresented to one ear and four stimuli at intervening carrier frequencieswere presented to the other ear for a group of subjects. The SSAEPstimuli were then reversed and applied to the opposite ear. The datashown in FIG. 9 are the latencies derived by modeling thevector-averaged phase delays measured across eight subjects in thesubject group. These phase delays were measured as having occurred after1 preceding cycle in the stimulus waveform. Phase delay was found to besimilar for SSAEP responses evoked by SSAEP stimuli having one testsignal component or SSAEP stimuli having more than one test signalcomponent, as long as adjacent carrier frequencies in the SSAEP stimuluswere separated by at least 1 octave. The experimental results showedthat phase (and hence latency) are stable over time and change asexpected with the intensity level of the stimulus. Phase delay was alsothe same for monaural and binaural presentation.

Since phase delay was found to be consistent, a normative database,stored within the master database 52, containing appropriate age and sexmatched normative phase delay or latency values for various stimuli maybe constructed. This normative database of phase delay values may thenbe used as a reference to detect abnormalities in subjects by measuringtheir phase delay or latency. In addition to absolute latency values,normative values for the differences between the latencies for responsesto pairs of stimuli may be useful in the detection of abnormal hearing.For instance, the difference between the latency estimate for SSAEPstimuli with 2000 and 4000 Hz carrier frequencies may be used.

The procedure for measuring latency uses the following steps:

a) Record the steady-state response to an SSAEP stimulus and measure theonset phase of the response in degrees;

b) Convert the onset phase to a phase delay (P) by subtracting it from360° (it may be necessary to make the phase delays ‘rational’ acrossdifferent carrier frequencies, i.e. an extra 360 degrees may be added tothe phase delay (i.e. phase unwrapping) so that the phase delay for acarrier frequency with a lower frequency is longer than the phase delayfor a carrier frequency with a higher frequency, (a situation whichmakes sense since higher frequencies are transduced near the basal endof the cochlea)); and,

c) Convert the phase delay to a latency (L) value in millisecondsaccording to the formula:

L=1000*(P+N*360)/(360*f _(m))  (7)

where f_(m) is the modulation frequency for the SSAEP stimulus thatevoked the SSAEP response and N is chosen as the number of precedingcycles. For modulation frequencies in the range of approximately 75-100Hz, N can be chosen to be 1. The value of N can be determined fromnormative studies using different modulation frequencies. N is set foreach response in order to bring the latency values calculated for asubject as close as possible to normative latency values.

Another way of determining the latency value is to present an SSAEPstimulus at a given carrier frequency and vary the modulation frequency.The phase of the response to each different modulation frequency is thenmeasured and plotted versus modulation frequency. The latency value isthen estimated from the slope of the phase versus modulation frequencyplot.

The objective audiometric test apparatus 10 can also perform AM/FMdiscrimination tests using SSAEP stimuli. The AM/FM discrimination testscorrelates with speech discrimination tests. One test involves using thenumber of responses to multiple IAFM SSAEP stimuli as an estimate of theability of a subject's auditory system to discriminate the frequenciesand intensities necessary for speech perception. If the SSAEP responsessuggest that the auditory system of the subject cannot make thesediscriminations then the subject will not be able to discriminate all ofthe words. Accordingly, the intensity of the SSAEP stimulus would besimilar to the intensity at which words would be presented during asubjective speech discrimination test in terms of root mean square SPL.

Referring to FIG. 9, the percentage of words that were correctlydiscriminated by a subject versus the number of significant SSAEPresponses that were detected when a multiple IAFM SSAEP stimulus waspresented to the subject is shown. The multiple IAFM SSAEP stimuluscomprised AM and FM signals with carrier frequencies of 500, 1000, 2000and 4000 Hz. The areas of the plotted circles are related to the numberof data points, with the largest circle representing 7 data points. Thisscattergram shows a correlation between the number of detected (i.e.significant) SSAEP responses and the word discrimination. The IAFMstimulus may be a good stimulus for looking at word discrimination sinceit presents multiple AM and FM stimuli simultaneously. Other multiplestimuli may also be used. For example, eight separate carrier signalsmay be presented with four of these amplitude-modulated and the otherfour frequency-modulated. The idea is to evaluate both amplitude andfrequency discrimination at multiple frequencies.

Accordingly, a test protocol that could be used to indicate the speechprocessing ability of the subject may consist of determining the numberof SSAEP responses that were evoked by a multiple IAFM SSAEP stimulus.The test period may persist for a certain amount of time, for example 12minutes, or until the residual noise background reached a minimal limit.The testing may be done both in the absence and presence of noisemasking as is conventionally done in subjective speech discriminationtests. The use of noise masking is important in testing subjects whohave difficulty listening to speech with background noise. The wordrecognition score is then be estimated from a function that correlatesthe number of detected SSAEP responses which were evoked by the multipleIAFM SSAEP stimulus to the word recognition score. The actual functionmay be determined from studies on a normative sample population ofsubjects. Control recordings need to be included to ensure thatresponses can be reliably recorded (and that the noise levels are nottoo high). These control tests may employ SSAEP response to single tones(i.e. sinusoids) with 100% amplitude modulation.

The AM/FM discrimination test could further comprise testing the abilityof the subject to discriminate frequency changes from amplitude changes.The amplitude of the SSAEP response to an FM component of an IAFM SSAEPstimulus could be compared to the amplitude of the SSAEP response to anAM component of an IAFM SSAEP stimulus in the form of a responseamplitude ratio (denoted by FM/AM). Alternatively individual FM SSAEPand AM SSAEP stimuli may be used. This FM/AM ratio can then be comparedto normative FM/AM ratios that may be computed for all age groups andstored in a database within the master database 52. Initial studiessuggest that FM/AM ratios for 500 to 6000 Hz carrier frequencies arebetween approximately 1 and 2 for younger subjects and belowapproximately 1 for older subjects. Deviations from this range may beused to indicate a problem in the part of the auditory system thatprocesses AM signals or the part of the auditory system that processesFM signals.

The objective audiometric test apparatus 10 can also perform ratesensitivity tests in which the amplitude of the SSAEP response to anSSAEP stimulus with increasing modulation frequencies may be measured.The various SSAEP stimuli that have been discussed (i.e. AM, FM, OVMM,MM, IAFM) and the use of exponential envelope modulation can bepresented with modulation rates that vary from a few Hz to severalhundred Hz. In general, as the modulation frequency (or modulation rate)increases, the amplitude of the SSAEP response to the SSAEP stimulusdecreases with the exception of local maxima that can occur in the 40,80, and 160 Hz ranges. However, this rate of response amplitude decreasecan vary for different subjects, particularly if the different subjectsinclude individuals with normal hearing and abnormal hearing.

Referring to FIG. 10, the SSAEP response amplitudes of an older subject100 measured in response to SSAEP stimuli having a range of modulationfrequencies is compared to the average SSAEP response amplitudesobtained from a group of normal control subjects 102. The data showsthat the decrease in SSAEP response amplitude with increasing modulationrate occurs more rapidly for the older subject 100 who also had a minorhearing loss. Although the older subject 100 could hear the SSAEPstimuli with higher modulation frequencies, the SSAEP responseamplitudes for the SSAEP stimuli with higher modulation frequencies didnot produce large amplitude SSAEP responses. Therefore, by comparing therate of decrease in the SSAEP response amplitude (i.e. the ratesensitivity) that occurs with increasing SSAEP stimuli with increasingmodulation rates to those obtained from an appropriate normativepopulation, subjects with abnormal hearing can be detected.Alternatively, instead of using the absolute value of the SSAEP responseamplitude (which is shown in FIG. 10), a ratio of SSAEP responseamplitudes may be used such as the ratio of the SSAEP response amplitudefor an SSAEP stimulus with a low modulation frequency to the SSAEPresponse amplitude for an SSAEP stimulus with a high modulationfrequency.

The larger decay in SSAEP response amplitude that occurs with increasingmodulation frequency in the older subject (with a minor hearing loss) issimilar to the decay that would be predicted by studies on older adultswho display large gap detection thresholds and more rapid decay in theirtemporal modulation transfer functions. Since an AM SSAEP stimulus witha carrier frequency of 100 Hz can be considered similar to a stimuluswhich is on for half a cycle and off for half a cycle then a 100 Hz AMSSAEP stimulus may be considered similar to having a stimulus on time of5 msec and a gap duration of 5 msec. Additionally, an AM SSAEP stimuluswith a 200 Hz carrier frequency is similar to having a stimulus “on”time of 2.5 msec, with a gap duration of 2.5 msec. Accordingly, theSSAEP responses which are recorded in response to SSAEP stimuli withmodulation frequencies in the range of 100 to 200 Hz may be used toprovide a physiological correlate of the modulation transfer function orthe gap function of an individual for gaps ranging from 5 msec to 2.5msec. The period of the gap of the SSAEP stimulus may be functionallyincreased by decreasing the modulation frequency or by increasing theexponent when using an exponential modulation signal with the SSAEPstimulus. Alternatively, both of these operations may be applied to theSSAEP stimulus.

The rate sensitivity test may further comprise using a multiple SSAEPstimulus comprising 4 AM test signals. The modulation frequencies of the4 AM test signals can initially be chosen to be 40, 44, 48, and 52 Hzfor example. If the two ears of a subject have been shown to havesimilar hearing ability, then the other ear of the subject may bepresented with a multiple SSAEP stimulus that comprises 4 AM testsignals with modulation frequencies of 42, 46, 50, and 54 Hz. After eachrecording is done, the modulation frequency of each AM test signal maybe increased by 10 Hz. In this manner, estimates of SSAEP responseamplitude may be measured in 10 Hz steps from approximately the 40 to190 Hz range. A plot of SSAEP response amplitude versus modulationfrequency may then be generated for each of the AM test signals or for acombination of the SSAEP responses to the 4 AM test signals such as themean SSAEP response amplitude at each modulation frequency. If the meanvalues at each of the modulation frequencies are found to be more usefulthan using information obtained for each of the 4 AM tones separately,then single rather than multiple stimuli may be used in this test.Alternatively, the carrier waveform could be band-limited noise that ismodulated at a single modulation rate. Since broadband noise can evoke alarger SSAEP response than that evoked by a single AM tone, the durationof this test should be shorter.

The objective audiometric test apparatus 10 may further estimate athreshold above which the auditory system of a subject no longerresponds to the modulation frequency used in the SSAEP stimulus. Thistest yields a “cutoff” modulation frequency threshold at which theauditory system of the subject no longer recognizes SSAEP stimuli withhigher modulation frequencies.

The objective audiometric test apparatus 10 may also be used withsubjects who have hearing aids. In this case the objective audiometrictest apparatus 10 may be used to adjust the settings (e.g. gain) of thehearing aid so that the subject can hear sounds near his/her threshold.The adjustment protocol comprises presenting the subject with an SSAEPstimulus while recording the EEG of the subject. The EEG is thenanalyzed to determine if an SSAEP response to the SSAEP stimulusoccurred. If an SSAEP response did not occur, then the gain of thehearing aid may be increased in step changes until an SSAEP response tothe SSAEP stimulus can be detected (or until some maximum level of gainis reached which is necessary to prevent the use of any gain levelswhich may damage the ear). Typically, this protocol would use SSAEPstimuli with an intensity level similar to that of conversational speechwhich may be approximately 50-60 dB HL. The SSAEP stimuli would also usemodulation depths that are typical of speech sounds such as FM testsignals having an FM depth of approximately 20% and AM test signalshaving an AM depth of approximately 50%.

Depending on the parameters that can be adjusted in the hearing aid, theadjustment protocol can be made more or less specific in its operations.For example, the gain of the hearing aid may be adjusted separately fordifferent frequency regions. Therefore, these gains may be separatelyand concurrently adjusted. Alternatively, if only the gain and thefilter slope of the hearing aid can be adjusted then a differentadjustment protocol may be used to adjust these parameters on the basisof the recognized SSAEP responses when presenting SSAEP stimuli atdifferent frequencies to the subject.

The objective audiometric test apparatus 10 may further include anotherhearing aid adjustment protocol known as the Seek and AdjustSingle-Multiple (SASM) technique. In this case, the gain of the hearingaid is automatically increased, by the objective audiometric testapparatus 10, until an SSAEP response to an SSAEP stimulus is detected.After this has been done for all the SSAEP stimuli, several paired SSAEPstimuli may then be presented to the subject. Alternatively, rather thanpairs, four or more AM test signal may be used in the multiple SSAEPstimulus. This is done because it has been shown that the thresholds fora multiple SSAEP stimulus (which may be more similar to natural soundssuch as speech) may be higher than the thresholds for SSAEP stimulihaving comprising single test signals. For example, in the case of ahigh frequency hearing loss that is steeply sloping near 4 kHz, both a 2kHz and a 4 kHz AM sinusoid may be presented together in a multipleSSAEP stimulus. If a 60 dB SPL stimulus needed to be amplified by 20 dBin order for the auditory system of the subject to detect the 2 and 4kHz AM test signals, then the multiple AM SSAEP stimulus may first bepresented to the subject using a 20 dB gain. If a significant SSAEPresponse is not obtained for either component of the multiple SSAEPstimuli, the gain for this frequency region in the hearing aid may beincreased a maximum of 3 times in +5 dB SPL steps for example. If, onthe other hand, a significant SSAEP response is not obtained for eithercomponent of the multiple SSAEP stimulus, then the gain of the hearingaid corresponding to the frequency region of either the first componentof the multiple SSAEP stimulus, or the second component of the SSAEPstimulus, or both components of the SSAEP stimulus may be increased amaximum of 3 times in +5 dB SPL steps (for example). In this manner,interactions can be evaluated, and the gain parameters that result inthe lowest overall gain will be automatically chosen for the hearingaid.

The objective audiometric test apparatus 10 may also be used toobjectively measure the audiometric thresholds of a subject bypresenting multiple SSAEP stimuli at multiple stimulus intensities tothe subject and recording the SSAEP responses. The objective audiometrictest apparatus 10 may then adjust the intensity levels of the SSAEPstimuli based on the detection of SSAEP responses and the amplitude ofthe SSAEP responses. Since a multiple SSAEP stimulus may be used,multiple audiometric thresholds may be estimated simultaneously.Experimental results have shown that audiometric threshold estimatedwith SSAEP stimuli are correlated with behavioral audiometricthresholds.

The objective audiometric threshold assessment method involves using amultiple SSAEP stimulus comprising 4 or more AM SSAEP test signalshaving an amplitude modulation depth of 100% and carrier frequenciesthat are separated by at least one-half octave. Alternatively, themultiple SSAEP stimulus may comprise FM test signals each having an FMmodulation depth in the range of 20%. The use of these modulation depthsfor the AM and FM components of the multiple SSAEP stimulus permit theaudiometric testing to be frequency specific. The SSAEP stimulus mayalso comprise MM or OVMM SSAEP test signals having similar modulationdepths. Furthermore, the SSAEP stimulus could also have an envelope thatis modulated by an exponential modulation signal with the modulationbeing done such that the resulting SSAEP stimulus is predominantlyfrequency specific.

The objective audiometric threshold assessment method may furtherinvolve adjusting the intensity of each component of the multiple SSAEPstimulus either independently or simultaneously. In the independentcase, the intensity of a component of the multiple SSAEP stimulus isreduced when the corresponding SSAEP response has been detected in therecorded EEG data. This independent intensity adjustment can be achievedif each component of the multiple SSAEP stimulus is sent to a separateDAC that can be adjusted in real time. Alternatively, the components ofthe multiple SSAEP stimulus may be combined digitally and then presentedthrough 2 DACs (1 for each ear of the subject). In this case, theintensity of a given component of the multiple SSAEP stimulus may bedigitally adjusted independently of the other components and combined inthe multiple SSAEP stimulus. This may be done provided that the DAC hassufficient resolution (e.g. 16 or more bits) to allow for the accuratepresentation of less intense components in the presence of higherintensity components.

To illustrate this principle, a multiple SSAEP stimulus may comprise 4AM test signals with carrier frequencies of 0.5, 1, 2, and 4 kHzpresented at 50 dB SPL. Each of the 4 test signals are represented in a16 bit buffer with about 16,384 bits each (this of course depends on theparticular data acquisition board that is used in the test apparatus).If the SSAEP response to the AM test signal with a carrier frequency of1 kHz becomes significant first, in order to present this AM test signalat 40 dB SPL while the other test signal components are presented at 50dB SPL, the test signal must be decreased by 10 dB SPL. Since themultiple SSAEP stimulus is stored in the RAM of the processor 12, a newmultiple SSAEP stimulus may be created by adding together the testsignal components (now with a reduced intensity AM test signal componenthaving a 1 kHz carrier frequency), and sent to the output buffer of theDAC 16. In this case, a dual buffer technique can be used in which theoutput buffer can be originally defined as being twice as long as anSSAEP stimulus. The multiple SSAEP stimulus is then loaded into thefirst half of the buffer. When a new multiple SSAEP stimulus is created,it can be loaded into the second half of the output buffer, such thatwhen the end of the first buffer is reached, the new multiple SSAEPstimulus can be seamlessly presented to the subject by simply changingthe memory address where the DAC 16 looks for data to convert intoanalog data.

Alternatively, rather than recreating the entire multiple SSAEPstimulus, the intensity of a single component within the multiple SSAEPstimulus may be adjusted using the Stimulus Flux method. The StimulusFlux method involves changing the intensity of a single component withinthe multiple SSAEP stimulus by creating and separately storing awaveform for each component of the multiple SSAEP stimulus. When theintensity of a particular component of the multiple SSAEP stimulus mustbe adjusted, the amplitude of the corresponding waveform is multipliedby the required amplitude factor such that when this waveform issubtracted from the multiple SSAEP stimulus, the intensity of thedesired component will be adjusted to the desired value. The newmultiple SSAEP stimulus may then loaded into the output buffer of theDAC 16 and subsequently presented to the subject 60.

An algorithm to carry out the objective audiometric threshold assessmentmethod may involve an adaptive staircase method. The adaptive staircasemethod is designed to bracket the audiometric threshold as efficientlyas possible by adjusting the value of a step size. The step size is usedto increase or decrease the intensity of components within the multipleSSAEP stimulus during subsequent presentations of the multiple SSAEPstimulus to the subject. The step size may be adjusted on the basis ofSSAEP response detection at a given stimulus intensity and thesequential replication of the audiometric threshold estimates. Onepossible definition of the audiometric threshold may be the intensity ofa particular component (for which the audiometric threshold is beingdetermined) of the multiple SSAEP stimulus between the last two stimulusintensities which resulted in detected and not detected SSAEP responseswhen using a minimum step size. The audiometric thresholds may be thenconfirmed based on replicated audiometric threshold estimates using thestaircase procedures shown in FIG. 11.

The objective audiometric threshold assessment method also includes theselection of initial stimulus intensity, initial step-size and minimumstep-size. Maximum and minimum limits for the stimulus intensities mustalso be set above and below which the method will not look foraudiometric thresholds. The rules for step size changes may also bedefined. Furthermore, the step-size itself may decrease with time orvary with the remaining range of stimulus intensities that are to betested (e.g. the step size can be defined as half of the distancebetween the present stimulus intensity and the minimum or maximumstimulus intensity which will be tested). There must also be a minimumstep size that will determine the precision whereby the audiometricthreshold is estimated (e.g this may be 5 or 10 dB SPL). Otherparameters that need to be set are the criteria whereby an SSAEPresponse is judged to be present or absent. The Phase weighted t-test orthe phase zone method may be used to detect the response. Alternatively,any detection method known in the art may be used. The criterion forjudging that an SSAEP response to an SSAEP stimulus is absent may beadjusted based on a minimum level of residual background noise in theprocessed EEG data. This criterion may also include a time limit forwhich the test expires. Alternatively, both of these criteria may beused.

The objective audiometric threshold assessment method may be implementedas follows. Several test signals components are combined in a multipleSSAEP stimulus which is presented at a given intensity level.Alternatively, a single test signal component may be used in the SSAEPstimulus. EEG data is simultaneously recorded, while the SSAEP stimulusis presented to the subject. The recorded EEG data is subsequentlyanalyzed for SSAEP response detection. As soon as one SSAEP response isdetected, the intensity of the test signal component (denoted as TS1)which evoked this SSAEP response is reduced by a step-size equal to halfthe dB SPL distance between the current intensity level for the testsignal component TS1 and the minimum intensity level which will betested in the audiometric threshold test. Accordingly, a new multipleSSAEP stimulus will be constructed based on this new test signalcomponent TS1. The method now involves the same steps as before:presenting the multiple SSAEP stimulus, recording EEG data and analyzingthe data for any SSAEP responses. The protocol also involves halving thestep size for the component in the multiple SSAEP stimulus that evoked adetected SSAEP response. Therefore, if an SSAEP response is detected forthe test signal component TS1, then the stimulus intensity for the testsignal component TS1 is reduced by the new step size. Alternatively, ifa response was not detected for the test signal component TS1, then theintensity level for the test signal component TS1 is increased by thenew step size. An estimate of noise level of the recorded EEG data mayalso be made to ensure that a lack of SSAEP response detection is notdue to excessive noise in the recorded EEG data. The noise estimate mayalso be used as a multiplicative factor to increase the test time whenpresenting a given multiple SSAEP stimulus. Thus, the test time may beextended as a function of the amount of noise in the recorded EEG data.

The objective audiometric threshold assessment method further comprisesobtaining threshold crossings until thresholds for all test signalcomponents in the multiple SSAEP stimulus have been obtained.Alternatively, when a sufficient number of threshold crossings have beenobtained for some test signal components in the multiple SSAEP stimulusbut not for other test signal components, a new multiple SSAEP stimuluscould be constructed comprising the test signal components for whichthresholds have not been obtained. Testing would then continue with thisnew multiple SSAEP stimulus.

The objective audiometric threshold assessment method may furthercomprise adjusting the intensities of all of the test signal componentsin the multiple SSAEP stimulus simultaneously. Some test signalcomponents of the multiple SSAEP stimulus may then be removed after aspecified duration of time if SSAEP responses have been detected forthese test signal components. The remaining test signal components inthe multiple SSAEP stimulus are then presented for another duration oftime such as 90 seconds for example. Since the detection of an SSAEPresponse to a single SSAEP stimulus may require about 90 seconds, thisaudiometric threshold detection procedure will be approximately 2 timesas fast as testing with an SSAEP stimulus which has single test signalcomponents. Additionally, since SSAEP stimuli comprising AM, FM, OVMM orMM test signals can be presented binaurally to a subject (i.e. to bothears of the subject), the objective audiometric threshold assessmentmethod may be 4 times as fast as testing with an SSAEP stimuluscomprising single test signals presented separately to each ear.

The objective audiometric threshold assessment method may alternativelypresent test signal components in the multiple SSAEP stimulus atdifferent intensities. Since SSAEP responses to test signals havingcarrier frequencies of 500 Hz and 4000 Hz typically require more time tobe detected compared to SSAEP responses to test signals having carrierfrequencies of 1000 and 2000 Hz, the stimulus intensity of the formertest signal components may be increased relative to the later testsignal components. Accordingly, the intensity of the test signalcomponents having 500 Hz and 4000 Hz carrier frequencies may bepresented at 10 dB SPL above the intensity level for the test signalcomponents having carrier frequencies of 1000 Hz and 2000 Hz (note thatthese exact frequencies do not have to be used and are shown forillustrative purposes; it is the frequency region in which they residethat is important). In this fashion, the SSAEP responses to the testsignal components may all be detected at approximately the same time.Alternatively, the multiple SSAEP stimulus may comprise only 2 testsignal components, such as test signal components having 500 Hz and 4000Hz carrier frequencies since these test signals will be transduced byseparate, fairly spaced apart regions of the basilar membrane and SSAEPresponses to these particular stimuli will both require long times todetected.

When the objective audiometric threshold assessment method has beencompleted, the results can be presented in a standard audiometric formatas is commonly known to those skilled in the art. The presentation ofthe test results may include highlighting whether SSAEP responses totest signal components were detected when the test signal componentswere presented alone or in combination with other test signalcomponents. For example, these SSAEP responses may be circled orhighlighted with a particular color. In addition to the actualaudiometric thresholds that were obtained from testing, estimates ofaudiometric thresholds may also be made which are extrapolated fromdetected SSAEP responses for test signal components which were presentedat higher stimulus intensities. For example, by taking the decrease inthe amplitude of the SSAEP responses obtained for a test signalcomponent presented at 60, 50, and 40 dB SPL, an estimate of when anSSAEP response will not be detected may be made by projecting a lineconnecting the amplitude of these detected SSAEP responses to the levelof the average background EEG noise.

Multi-modality Testing

Referring to FIG. 12, an alternate embodiment of the objectiveaudiometric test apparatus 10 comprises an objective multi-modality testapparatus 200. Please note that in FIG. 12, like numerals were used torepresent elements that are similar to the elements of the objectiveaudiometric test apparatus 10 shown in FIG. 1a. The objectivemulti-modality test apparatus 200 may be used to concurrently test othermodalities while the auditory system of the subject 60 is being tested.In this embodiment, the visual and somatosensory modalities areconcurrently tested with the auditory modality. In other embodiments,other sensory modalities may be concurrently tested with the auditorysystem. The testing of multiple modalities may allow for thedetermination of whether an auditory abnormality is part of a morewidespread disorder of the nervous system such as multiple sclerosis forexample. In addition, multi-modality testing may be used to investigateneurological disorders.

The objective multi-modality test apparatus 200 comprises the processor12, a data acquisition board 202 having at least one DAC 204 and atleast one ADC 206, amplifiers 22, 208 and 210, filters 24, 212 and 214,transducers 26, 216 and 218, sensors 28, 220 and 224, amplifiers 30, 224and 226 and filters 32, 228 and 230. The processor 12 further comprisesa software program 232 that encodes the functionality of the objectivemulti-modality test apparatus 200. The software program 232 comprises asignal creator 234, a modulator 236 and an analysis module 238 having anoise reduction module 240 and a detection module 242. The softwareprogram 232 is also coupled to a plurality of a master database 250which comprises a plurality of databases D1 to Dn. The processor 12 isalso coupled to the storage device 34 and the computer display 36.

In use, the signal creator 234 generates test signals that areappropriate as stimuli for evoking auditory, visual and somatosensoryresponse potentials. The modulator 236 may be employed in the creationof the test signals. The test signals are then sent to the DAC 204 whichmay comprise a plurality of output channels or may be a plurality ofsingle channel DACs. The DAC 204 sends the test signals to theamplifiers 22, 208 and 210. The amplifiers 22, 208 and 210 amplify thetest signals to adjust the intensity level of the test signals to levelsthat are suitable for testing. The amplified test signals are then sentto filters 24, 212 and 214 to remove any noise from the digital toanalog conversion process and the amplifying process.

The filtered, amplified test signals are then sent to the transducers26, 216 and 218 so that the test signals may be transduced by andsimultaneously presented to the subject 60. The transducer 26 may be anauditory transducer that transduces the appropriate test signal into anauditory stimulus. Accordingly, the transducer 26 may be a pair ofheadphones or at least one insert earphone. The particular test signalsused for the auditory stimulus can be any of the stimuli that werepreviously discussed for the objective audiometric test apparatus 10.For instance the auditory stimulus may be two AM sinusoids (i.e. tones)at different carrier frequencies, having modulation frequencies of 87and 93 Hz.

The transducer 216 may be a visual transducer that transduces theappropriate test signal into a visual stimulus. Accordingly, thetransducer 216 may be a strobe light which can produce a pulsating flashor the transducer 216 may be a grid of light emitting diodes. The visualstimuli may be presented at modulation rates of 16 and 18 Hz, forexample, to the left and right eyes of the subject 60. Alternatively,the visual stimuli may be presented to only one eye of the subject 60.As in the case of the auditory stimuli, multiple modulated visualstimuli may be presented to a single eye.

The transducer 218 may be a tactile transducer that transduces theappropriate test signal into a tactile stimulus. Accordingly, thetransducer 218 may be a vibrotactile stimulator or the like. Thevibrotactile stimulator may be applied to at least one finger of thesubject 60. For instance, the vibrotactile stimulator may be applied tothe left and right index fingers of the subject 60. The tactile stimulimay be presented at rates of 23 and 25 Hz although other presentationrates may be used.

When presenting the multi-modality stimulus to the subject 60, the testsignals which are used must be chosen such that the frequencies of theresponses to each of the auditory, visual and tactile stimuli are notequal to each other, are not integer multiples of each other and share aminimum of common factors. This must be ensured so that the responsesand their harmonics do not interfere with one another.

EEG data is recorded while the multi-modality stimulus is presented tothe subject 60. The EEG data of the subject 60 is recorded using sensors28, 220 and 222 that are typically electrodes. These electrodes areplaced on certain regions of the subject 60 to obtain EEG data withbetter signal to noise ratios. The sensors 220 that measure the responseto the visual stimulus may be placed over the occipital regions of thebrain of the subject 60. The sensors 222 that measure the response tothe tactile stimulus may be placed over the central scalp which iscontralateral to the presentation of the tactile stimulus. The sensors28 that measure the response to the auditory stimulus may be placed onthe vertex of the subject 60. In each of these cases alternativeplacements of the electrodes is possible. For instance, the placement ofthese sensors may also involve using a plurality of electrodes placed atnumerous (i.e. 32 or 64) locations on the scalp of the subject 60.

Each of the sensed potentials (i.e the EEG data which is also understoodto be a time series data) may then be amplified by amplifiers 30, 224and 226 to an amplitude level that is sufficient for digitization. Theamplified EEG data may then be filtered by filters 32, 228 and 230. Theamplified, filtered EEG data is then sent to the ADC 206 fordigitization at a sampling rate that is sufficient to sample the EEGdata without aliasing.

The sampled data is then analyzed by the analysis module 242. The datais first preprocessed by the noise reduction module 240 to reduce theamount of noise in the sampled data and produce noise reduced EEG data.The noise reduction module 242 may use the sample weighted averagingmethod to reduce noise. The noise reduction module 242 may also useadaptive artifact rejection. Alternatively, the noise reduction module242 may use any noise reduction algorithm that is known in the art.

The noise reduced EEG data is then analyzed by the detection module 242to determine whether there are any responses present in the noisereduced EEG data. The detection module 242 may implement the phaseweighted t-test, the phase zone technique or the MRC method.Alternatively, the detection module 242 may use other detectionalgorithms that are known in the art. The detection module 242 may alsoprovide a probability estimate that a detected response is truly asignal and not noise. The detected responses may then be compared tonormative data on other subjects which is contained in the masterdatabase 250. This comparison may provide an indication of whether thesubject 60 has a widespread disorder of the neural system.

Based on the objective multi-modality test apparatus 200, a procedurefor multi-modality testing would comprise the following steps:

(i) Attach electrodes to the subject 60;

(ii) Set up or attach transducers to present the multi-modality stimulito the subject 60;

(iii) Present the multi-modality stimuli to the subject 60 wherein thestimulus for each modality is synchronized with the objectivemulti-modality test apparatus 200 so that the multiple responses can berecognized by their signature modulation frequencies;

(iv) Record the EEG data at each electrode location to obtain three EEGdata time series;

(iv) Reduce the noise in each EEG data time series to produce a set ofnoise reduced EEG data time series;

(v) Detect the steady-state responses in each noise-reduced EEG datatime series wherein a steady-state response is recognized at themodulation frequency specific to the modality stimulus that evoked theresponse; and,

(vi) Compare the amplitudes of the detected responses to normativevalues matched for age and sex (alternatively the comparison can be donewithin the subject 60 if modality stimuli are presented to both the leftand right sides of the body of the subject 60).

Portable Objective Multi-modality Test Apparatus

The present invention further comprises a portable objectivemulti-modality test apparatus 300 as shown in FIG. 13. The portableobjective multi-modality test apparatus 300 is similar to the objectivemulti-modality test apparatus 200 that was shown in FIG. 12 andtherefore comprises many of the same components. The portable objectivemulti-modality test apparatus 300 comprises a laptop computer 302 whichhas a screen 304, a software program 306, a storage device 308, a masterdatabase 390 comprising a plurality of databases D1 to Dn and a PCMCIAdata communication card 310. The software program 306 comprises thesignal creator 234, the modulator 236 and the analysis module 238including the noise reduction module 240 and the detection module 242.The objective multi-modality test apparatus 300 further comprises acontrol box 312 having amplifiers 314, 316, 318, 320, 322, 324, 326 and328, filters 330, 332, 334, 336, 338, 340 and 342, transducers 350, 352,354, 356, 358 and 360 and sensors 362, 364 and 366.

The portable objective multi-modality test apparatus 300 operates inmuch the same manner as the objective multi-modality test apparatus 200except that the apparatus is based on the laptop 302 and the control box312. Alternatively, a palmtop or other portable computing device may beused. On the screen 304 there are various graphical user interfaces(GUI) windows that are implemented by the software program 306. There isa Load protocol window 370, a View Stimuli window 372, a View EEG window374, a data acquisition window 376, a continue acquisition window 378and a process data window 380. These GUI windows allow a user to operatethe portable objective multi-modality test apparatus 300 and performvarious tests on the subject 60.

The PCMCIA data communication card 310, which may be a NationalInstrument DAQCard-6062e card, enables functional communication and datatransfer between the laptop 302 and the control box 312. The PCMCIA datacommunication card 310 provides test signals to the control box 312 andreceives the recorded EEG data. Other communication systems may also beused to support data transfer between the components of the systemprovided that they can support the necessary data transfer rates.

The control box 312 contains two audio amplifiers 314 and 316 whichamplify the test signals, provided by the PCMCIA data communication card310, and provide the test signals to two transducers 350 and 352 whichtransduce the test signals into acoustic stimuli and present theacoustic stimuli to the subject 60. The laptop 302 controls theintensity of the acoustic stimulus via the gain of the audio amplifiers314 and 316. Alternatively, eight or more audio amplifiers may becontained in the control box 312 in order to permit separate intensitycontrol of test signals contained within a multiple SSAEP stimulus thatmay be presented to the subject 60. The output of the audio amplifiers314 and 316 are then sent to filters 330 and 332 to remove anydigitization noise or artifacts in the test signals. The filters 330 and332 may also be used to introduce pass band masking signals into thetest signals. In a like manner, the control box 312 further comprisesother amplifiers 318 and 320 and filters 334 and 336 that manipulate thetest signals before they are transduced into visual and tactile stimuli.

The various test signals are then transduced by the transducers 350,352, 354 and 356. The transducers 352 and 354 may be headphones orinsert earphones. The transducers 352 and 354 may also be speakers ifthe ears are not to be tested separately. The transducer 354 providesthe visual stimuli to the subject 60 and may be a strobing light sourceor goggles that can be placed over the eyes of the subject 60. Thetransducer 356 provides the tactile stimuli to the subject 60 and may beat least one vibration transducer that is attached to at least onefinger of the subject 60. Each stimulus in each modality is modulated ata unique frequency. Each stimulus must also be synchronously initiatedand locked to the portable objective multi-modality test apparatus 300so that the steady-state responses to the multi-modality stimulus can berecognized by their signature modulation frequencies (the steady-stateresponses occur at the modulation frequency used in the modalitystimulus).

To record the steady-state responses to each of the modality stimuli,groups of electrodes 362, 364 and 366 are placed on the scalp of thesubject 60 as was previously described for the objective multi-modalitytest apparatus 200. Multi-modality steady-state testing could also beachieved by using multiple scalp recordings (e.g. using 32 electrodelocations over the scalp) or by recording the EEG data with a smallnumber of input data channels (e.g., 3) with specifically locatedelectrodes.

The electrodes 362, 364 and 366 provide sets of EEG time series data tothe control box 312. These sets of EEG time series data are thenamplified by the amplifiers 324, 326 and 328 and filtered by the filters338, 340 and 342. Alternatively only one amplifier and one filter may beused. The amplifiers 324, 326 and 328 have gain settings which are underthe control of the laptop 302. The filters 338, 340 and 342 may beprogrammable analog filters for lowpass, highpass, and notch filteringthe sets of EEG time series data. These filters 338, 340 and 342 mayalso be controlled by the laptop 30 (the laptop may also control theother amplifiers and filters within the control box 312). The filteredand amplified sets of EEG time series data are then sent to the PCMCIAdata communications card 310 where the sets of EEG time series data aredigitized and sent to the analysis module 238. The sets of EEG timeseries data can then be processed to remove noise, via the noisereduction module 240 and then analyzed for response detection via thedetection module 242 as was previously described for the objectivemulti-modality test apparatus 200.

The portable objective multi-modality test apparatus 300 may furthercomprise means to enable the adjustment of hearing aid devices which maybe worn by the subject 60. In particular, the control box 312 may beadapted to communicate with hearing aid devices 358 and 360. Thecommunication may be via a physical connection such as a ribbon cable361. Alternatively, in the case of implanted hearing aid devices, thecommunication may occur via RF telemetry as is used to adjust otherimplanted biomedical devices (such as implanted stimulators). Theportable objective multi-modality test apparatus 300 may then be used toadjust the frequency specific gain settings, the filter slope setting orother relevant settings of the hearing aid devices 358 and 360 asdescribed previously in the method of adjusting hear aids using SSAEPstimuli. The adjustment of the hearing aid devices 358 and 360 may bedone by a trained medical professional or may be adjusted automaticallyby the laptop 302 based upon the pass/fail results of the SSAEP testingprocedure.

It should be understood that various modifications can be made to thepreferred embodiments described and illustrated herein, withoutdeparting from the present invention, the scope of which is defined inthe appended claims.

What is claimed is:
 1. A method of testing the hearing of a subject,said method comprising the steps of: (a) selecting at least one testsignal; (b) modulating at least one of the amplitude and frequency ofsaid at least one test signal by an exponential modulation signal toproduce at least one modulated test signal; (c) transducing said atleast one modulated test signal to create an acoustic stimulus andpresenting said acoustic stimulus to said subject; (d) sensingpotentials from said subject while substantially simultaneouslypresenting said acoustic stimulus to said subject; and, (e) analyzingsaid potentials to determine whether said potentials comprise dataindicative of the presence of at least one steady-state response to saidacoustic stimulus.
 2. The method of claim 1, wherein said exponentialmodulation signal comprises a sinusoidal signal having an exponent whichis greater than 1 and less than or approximately equal to
 4. 3. Themethod of claim 1, wherein said exponential modulation signal is definedby the formula: y[n]=2*m _(a)*[(((1+sin(2*π*f _(am)*t*n))/2)^(eam)−0.5)+1] in which: y[n] is a time series of data pointsrepresenting said exponential modulation signal; m_(a) is the AMmodulation depth; f_(am) is the AM modulation frequency; t is the timespacing between said data points; n is a given data point; and, eam isthe exponent of said exponential modulation signal.
 4. The method ofclaim 1, wherein steps (b) and (e) of said method are effected by asoftware program.
 5. A method of testing the hearing of a subject,wherein said method comprises the steps of: (a) creating anoptimum-vector mixed modulation test signal comprising at least onesignal having an amplitude modulated component with a first phase and afrequency modulated component with a second phase wherein said secondphase is adjusted relative to said first phase to evoke an increasedresponse from said subject; (b) transducing said test signal to createan acoustic stimulus and presenting said acoustic stimulus to saidsubject; (c) sensing potentials from said subject while substantiallysimultaneously presenting said acoustic stimulus to said subject; and,(d) analyzing said potentials to determine whether said potentialscomprise data indicative of the presence of at least one steady-stateresponse to said acoustic stimulus.
 6. The method of claim 5, whereinsaid method further comprises creating a database of normative optimalphase difference data correlated to subject characteristics and stimuluscharacteristics.
 7. The method of claim 5, wherein the method furthercomprising using said database of normative phase difference data toadjust said second phase relative to said first phase.
 8. A method oftesting the hearing of a subject, wherein said method comprises thesteps of: (a) creating a test signal comprising at least one independentamplitude modulated and frequency modulated signal having an amplitudemodulated component and a frequency modulated component, wherein saidamplitude modulated component comprises a first modulation frequency anda first carrier frequency and said frequency modulated componentcomprises a second modulation frequency and a second carrier frequencywherein said first modulation frequency is substantially different fromsaid second modulation frequency and said first carrier frequency issubstantially similar to said second carrier frequency; (b) transducingsaid test signal to create an acoustic stimulus and presenting saidacoustic stimulus to said subject; (c) sensing potentials from saidsubject while substantially simultaneously presenting said acousticstimulus to said subject; and, (d) analyzing said potentials todetermine whether said potentials comprise data indicative of asteady-state response to each amplitude modulated component and asteady-state response to each frequency modulated component.
 9. Anapparatus for testing the hearing of a subject, wherein said apparatuscomprises: (a) a signal creator adapted to create a test signalcomprising at least one combined amplitude modulated and frequencymodulated signal having an amplitude modulated component with a firstphase and a frequency modulated component with a second phase whereinsaid signal creator comprises means for adjusting said second phaserelative to said first phase; (b) a transducer electrically coupled tosaid processor and adapted to transduce said test signal to create anacoustic stimulus and present said acoustic stimulus to said subject;(c) a sensor adapted to sense potentials from said subject while saidacoustic stimulus is substantially simultaneously presented to saidsubject; and, (d) a processor electrically coupled to said sensor andadapted to receive said potentials and analyze said potentials todetermine if said potentials comprise data indicative of at least oneresponse to said acoustic stimulus.
 10. The apparatus of claim 9,wherein said apparatus further comprises a database of normative phasedifferences data correlated to subject characteristics and stimuluscharacteristics.
 11. The apparatus of claim 9, wherein said processor isfurther adapted to create a second test signal comprising at least oneindependent amplitude modulated and frequency modulated signal.
 12. Amethod of analyzing potentials to determine whether said potentialscomprise data indicative of the presence of at least one steady-stateresponse to a steady-state evoked potential stimulus, wherein saidmethod comprises the steps of: (a) presenting an evoked potentialstimulus to a subject; (b) sensing potentials from said subject whilesubstantially simultaneously presenting said stimulus to said subject toobtain a plurality of data points; (c) transforming said plurality ofdata points into a second plurality of data points; (d) biasing saidsecond plurality of data points with an expected phase value to obtain aplurality of biased data points; and, (e) applying a statistical test tosaid plurality of biased data points to detect said response.
 13. Themethod of claim 12, wherein effecting steps (c) and (d) comprises thesteps of: (f) forming a plurality of sweeps from said plurality of datapoints; (g) averaging said plurality of sweeps to obtain a plurality ofaveraged data points; (h) calculating a plurality of Fourier componentsfor said plurality of averaged data points; (i) calculating theamplitude (a_(i)) and phase (θ_(I)) for said plurality of Fouriercomponents; (j) biasing said amplitudes (a_(i)) to obtain biased datapoints (p_(i)) according to the formula: p _(i) =a _(i)*cos(θ_(I)−θ_(e))wherein θ_(e) is said expected phase value.
 14. The method of claim 12,wherein effecting step (e) comprises the steps of: (k) calculating upperconfidence limits using a one tailed Student t-test on biased amplitudeswhich represent noise in the vicinity of Fourier components where saidresponse should occur; and, (l) comparing biased amplitudes of Fouriercomponents where said response should occur to said upper confidencelimits to determine if said biased amplitudes are larger than said upperconfidence limit.
 15. The method of claim 12, wherein said expectedphase value is obtained from a database of normative expected phasevalues correlated to subject characteristics and stimuluscharacteristics.
 16. The method of claim 12, wherein said expected phasevalue is obtained from previous testing on said subject using stimulihaving a higher intensity.
 17. The method of claim 12, wherein saidstimulus contains other components for which responses have beendetected and said expected phase value is obtained from extrapolation ofsaid phase values for said responses which have already been detected.18. A method of detecting a response to an evoked potential stimulus,wherein said method comprises the steps of: (a) presenting an evokedpotential stimulus to a subject; (b) sensing potentials from saidsubject while substantially simultaneously presenting said stimulus tosaid subject to obtain a plurality of data points; and, (c) calculatingphase values for said plurality of data points, wherein, a response isdetected if an adequate number of said calculated phase values fallwithin a predetermined phase value range.
 19. The method of claim 18,wherein said effecting step (c) further comprises the steps of: (d)separating said plurality of data points into epochs; (e) calculating aFourier component for the frequency at which said response should occurfor each epoch; and, (f) calculating the phase of each Fourier componentcalculated in step (e).
 20. The method of claim 19, wherein said methodfurther comprises the steps of: (g) calculating a target phase range;(h) calculating the number of said Fourier components that have a phasethat is within the target phase range (N); and, (i) using binomialanalysis with N to determine whether said plurality of data pointscontain a response.
 21. The method of claim 20, wherein said targetphase range is calculated based on a database of normative expectedphases correlated to subject characteristics and stimuluscharacteristics.
 22. An apparatus for testing the hearing of a subject,wherein said apparatus comprises: (a) a signal creator adapted to createa test signal; (b) a transducer electrically coupled to said signalcreator and adapted to transduce said test signal to create an acousticstimulus and present said acoustic stimulus to said subject; (c) asensor adapted to sense potentials from said subject while said acousticstimulus is substantially simultaneously presented to said subject; and,(d) a processor electrically coupled to said sensor and adapted toreceive said potentials and analyze said potentials to determine if saidpotentials comprise data indicative of at least one response to saidacoustic stimulus; wherein said analysis involves biasing saidpotentials based on an expected phase value.
 23. The apparatus of claim22, wherein said apparatus further comprises a database of expectedphase value data correlated to subject characteristics and stimuluscharacteristics.
 24. A method of noise reduction for a plurality of datapoints which are obtained during steady-state evoked potential testing,wherein said plurality of data points comprise at least one signal andnoise and wherein said method comprises the steps of: (a) obtaining saidplurality of data points; (b) separating said plurality of data pointsinto a plurality of epochs; and, (c) applying an adaptive noisereduction method to each epoch.
 25. The method of claim 24, wherein saidadaptive noise reduction method comprises effecting sample weightedaveraging wherein weights are calculated based on the variance of noise,wherein the frequency of the noise is in close proximity to thefrequency of the signal.
 26. The method of claim 25, wherein said sampleweighted averaging is effected according to the steps of: (d) forming aplurality of sweeps by concatenating the epochs together; (e) filteringeach sweep to obtain a plurality of filtered sweeps; (f) aligning eachsweep to form a first matrix in which the sweeps are the rows of thematrix and the epochs within the plurality of sweeps are the columns ofthe matrix and aligning each filtered sweep in a similar fashion to forma filtered matrix which is used to calculate weights; (g) calculatingthe variance of each epoch in the filtered matrix to obtain a noisevariance estimate for each epoch in the filtered matrix; (h) normalizingthe noise variance estimate for each epoch in the filtered matrix bydividing the noise variance estimate for each epoch in the filteredmatrix by the sum of all noise variance estimates for the epochs alongthe column of the filtered matrix which contains the epoch to obtain anormalized noise variance estimate for each epoch; (i) inverting eachnormalized noise variance estimate to obtain a weight for each epoch andmultiplying each corresponding epoch in the first matrix by itsrespective weight to obtain a plurality of weighted epochs; and, (j)summing all of the weighted epochs in the first matrix along the columnsof the first matrix to obtain a signal estimate.
 27. The method of claim26, wherein filtering comprises using a bandpass filter having apassband region which is local to the frequency region where the atleast one signal resides such that the calculated weights are based onthe frequency region local to the frequency region where the at leastone signal resides.
 28. A method of objectively testing the hearing of asubject, wherein said method comprises the steps of: (a) selecting anauditory test; (b) creating an appropriate test signal comprising atleast one component for said auditory test; (c) transducing said testsignal to create a stimulus and presenting said stimulus to saidsubject; (d) sensing potentials from said subject while substantiallysimultaneously presenting said stimulus to said subject; and, (e)analyzing said potentials to detect at least one response.
 29. Themethod of claim 28, wherein said auditory test is a latency test whichcomprises calculating a latency value for a detected response andcomparing said latency value to a database of normative latency valuesto obtain an indication of the normal/abnormal status of the auditorysystem of said subject.
 30. The method of claim 29, wherein said latencytest comprises the steps of: (f) calculating an onset phase for eachdetected response in said sensed potentials; (g) calculating a phasedelay (P) for each detected response by subtracting each onset phasefrom 360 degrees; (h) calculating a latency value (L) for each detectedresponse according to the following formula: L=1000*(P+N*360)/(360*f_(m)) where N is the number of cycles that have occurred in the stimulusbefore each detected response to a respective component of said stimulusoccurs and f_(m) is the frequency of said detected response; and, (i)comparing each latency value to a database of normative latency valuesto obtain an indication of the normal/abnormal status of the auditorysystem of said subject.
 31. The method of claim 30, wherein N isapproximately 1 and fm is in the range of approximately 75 to 100 Hz orin the range of approximately 150 to 190 Hz.
 32. The method of claim 29,wherein said latency test further comprises the steps of: (j)calculating a first latency value for a first detected response andcalculating a second latency value for a second detected response; (k)finding the difference between said first latency value and said secondlatency value to obtain a differential latency value; and, (l) comparingsaid differential latency value to a database of normative differentiallatency values to obtain an indication of the status of the auditorysystem of said subject.
 33. The method of claim 28, wherein saidauditory test is an aided hearing test which comprises the followingsteps: (m) fitting said subject with a hearing aid; (n) performing steps(b) to (e); (o) adjusting the hearing aid if an inadequate number ofresponses are detected in step (e); and, (p) performing steps (n) and(o) until an adequate number of responses have been detected in saidpotentials.
 34. The method of claim 33, wherein said method furthercomprises effecting step (o) by adjusting the gain of said hearing aidfor a frequency region which is substantially similar to the frequencyregion of a component in said test signal for which a response was notdetected.
 35. The method of claim 28, wherein said auditory test is anAM/FM discrimination test and said test signal comprises at least oneindependent amplitude modulation and frequency modulation signal,wherein, said AM/FM discrimination test comprises the steps of: (q)performing steps (b) to (e); (r) calculating a response ratio accordingto the number of detected responses divided by the total number ofresponses which could have occurred in response to said stimulus; and,(s) estimating a word recognition score correlated to said responseratio.
 36. The method of claim 35, wherein said test signal furthercomprises noise masking.
 37. The method of claim 28, wherein saidauditory test is an AM/FM discrimination test and said test signalcomprises an amplitude modulated component and a frequency modulatedcomponent, wherein, said AM/FM discrimination test comprises the stepsof: (t) performing steps (b) to (e); (u) calculating a first amplitudeof a response to said frequency modulated component; (v) calculating asecond amplitude of a response to said amplitude modulated component;(x) calculating an amplitude ratio by dividing said first amplitude bysaid second amplitude; and, (y) comparing said amplitude ratio to adatabase of normative amplitude ratios to obtain an indication of thestatus of the auditory system of said subject.
 38. The method of claim37, wherein said test signal comprises a plurality of pairs of amplitudemodulated components and frequency modulated components and the methodfurther comprises carrying out steps (u) to (y) for each of said pairs.39. The method of claim 28, wherein said auditory test is a depthsensitivity test and said test signal comprises one or more amplitudemodulated components having a modulation depth or one or more frequencymodulated components having a modulation depth, wherein, said depthsensitivity test comprises the steps of: (z) carrying out steps (b) to(e); (aa) obtaining an estimate of response amplitude for each detectedresponse; and, (bb) comparing each estimated response amplitude with adatabase of normative response amplitudes to obtain an indication of thestatus of the auditory system of said subject.
 40. The method of claim39, wherein said depth sensitivity test further comprises the steps of:(cc) detecting responses to test signals comprising a first modulatedcomponent having a first modulation depth and a second modulatedcomponent having a second modulation depth, wherein said modulationdepths are different; (dd) analyzing said detected responses to obtainan estimate of a first response amplitude for a detected response tosaid first modulated component and a second response amplitude for adetected response to said second modulated component; (ee) calculating aresponse amplitude ratio from said first response amplitude and saidsecond response amplitude; and, (ff) comparing said response amplituderatio with a database of normative response amplitude ratios to obtainan indication of the status of the auditory system of said subject. 41.The method of claim 28, wherein said auditory test is a rate sensitivitytest which comprises the following steps: (gg) creating a set ofmodulation frequencies comprising at least two modulation frequencies;(hh) creating a set of test signals wherein each test signal comprisesat least one amplitude modulated component having a unique modulationfrequency chosen from said set of modulation frequencies; (ii) carryingout steps (c) and (d) for each test signal in said set of test signals;(jj) analyzing said set of potentials to detect a response for eachamplitude modulated component in said set of test signals and estimatinga response amplitude for each detected response; (kk) calculating a ratesensitivity value based on said estimated response amplitudes; and, (ll)comparing said rate sensitivity value to a database of normative ratesensitivity values to obtain an indication of the status of the auditorysystem of said subject.
 42. The method of claim 41, wherein said ratesensitivity value is the slope of the line resulting from a plot ofestimated response amplitude for each amplitude modulated componentversus modulation frequency for each amplitude modulated component. 43.The method of claim 28, wherein said auditory test is a supra-thresholdtest comprising an intensity limen test and said test signal comprisesan amplitude modulated component having a modulation depth ofapproximately 100%, wherein said intensity limen test comprises thesteps of: (mm) performing steady state evoked potential testing whileminimizing the modulation depth of the test signal upon each detectedresponse to determine a minimum modulation depth at which a response isdetected; and, (nn) comparing said minimum modulation depth with adatabase of normative minimum modulation depths to obtain an indicationof the status of the auditory system of said subject.
 44. The method ofclaim 43, wherein said supra-threshold test further comprises afrequency limen test and said test signal comprises an amplitudemodulated component having a frequency modulation depth, wherein, saidfrequency limen test comprises the steps of: (oo) determining a minimummodulation depth at which a response is detected; and, (pp) comparingsaid minimum modulation depth with a database of normative minimummodulation depths to obtain an indication of the status of the auditorysystem of said subject.
 45. The method of claim 28, wherein saidauditory test is an auditory threshold test and said test signalcomprises two or more combined amplitude modulation and frequencymodulation signals having carrier frequencies which are separated bymore than one-half octave, wherein, each combined amplitude modulationand frequency modulation signal has a frequency modulated component andan amplitude modulated component and the phase of the frequencymodulated component is adjusted relative to the phase of the amplitudemodulated component, wherein said auditory threshold test comprises thestep of: (qq) carrying out steps (c) to (e) to determine a minimalstimulus intensity for which a response is detected for each combinedamplitude modulation and frequency modulation signal.
 46. The method ofclaim 45, wherein the envelope of each combined amplitude modulation andfrequency modulation signal is modulated by an exponential modulationsignal.
 47. The method of claim 45, wherein said auditory threshold testis conducted for a maximum time limit which is adjusted depending on theamount of noise in said potentials.
 48. The method of claim 45, whereinsaid auditory threshold test comprises upwardly adjusting theintensities of components in said stimulus which tend to evoke responseshaving smaller amplitudes, so that all components in said stimulus evokeresponses having similar amplitudes.
 49. The method of claim 45, whereinstep (e) comprises the step of: (rr) reducing noise in said potentialsto obtain reduced noise potentials by employing sample weightedaveraging.
 50. The method of claim 45, wherein step (e) comprises thestep of: (ss) performing a phase weighted t-test on said potentials todetermine whether said potentials comprise data indicative of at leastone response to said stimulus.
 51. The method of claim 45, wherein step(e) comprises the step of: (tt) performing a modified Rayleigh test ofcircular uniformity (MRC) on said potentials to determine whether saidpotentials comprise data indicative of at least one response to saidstimulus.
 52. The method of claim 45, wherein adjusting said intensityof one of said components comprises using the Stimulus Flux method whichcomprises creating another waveform of appropriate phase and amplitude,which when added to said test signal, results in the desired adjustmentof said intensity of one of said components.
 53. An apparatus forobjectively testing the hearing of a subject, wherein said apparatuscomprises: (a) a selector adapted for selecting an auditory test toperform on said subject; (b) a signal creator electrically coupled tosaid selector and adapted to create an appropriate test signalcomprising at least one component for said test; (c) a transducerelectrically coupled to said signal creator and adapted to transducesaid test signal to create an acoustic stimulus and present saidacoustic stimulus to said subject; (d) a sensor adapted to sensepotentials from said subject while said acoustic stimulus issubstantially simultaneously presented to said subject; (e) a processorelectrically coupled to said sensor and adapted to receive saidpotentials and analyze said potentials to determine if said potentialscomprise data indicative of at least one response to said acousticstimulus; and, (f) a programmable hearing aid coupled to said processor,wherein, said programmable hearing aid comprises a plurality ofprogrammable gain factors for different frequency regions and at leastone programmable filter slope.
 54. A method of testing at least twosenses of a subject, wherein said method comprises the steps of: (a)selecting a first steady-state test signal to test a first sensorymodality; (b) transducing said first steady-state test signal to createa first stimulus and presenting said first stimulus to said subject; (c)selecting a second steady-state test signal to test a second sensorymodality; (d) transducing said second steady-state test signal to createa second stimulus and presenting said second stimulus to said subject;(e) sensing potentials while substantially simultaneously presentingboth stimuli to said subject; and, (f) analyzing said potentials todetermine whether said potentials comprise data indicative of at leastone steady-state response to said stimuli.
 55. The method of claim 54,wherein said steady-state test signals comprise an auditory signal, avisual signal and a tactile signal.
 56. The method of claim 54, whereinsaid method further comprises ensuring that all modulation frequenciesin the stimuli are not integer multiples of one another.
 57. Anapparatus for testing at least two senses of a subject, wherein saidapparatus comprises: (a) a signal creator adapted to create a firststeady-state test signal and a second steady-state test signal; (b) afirst transducer electrically coupled to said selector and adapted totransduce said first test signal to create a first stimulus and presentsaid first stimulus to said subject; (c) a second transducerelectrically coupled to said selector and adapted to transduce saidsecond test signal to create a second stimulus and present said secondstimulus to said subject; (d) a first sensor adapted to sense firstpotentials from said subject while said first stimulus is substantiallysimultaneously presented to said subject; (e) a second sensor adapted tosense second potentials from said subject while said second stimulus issubstantially simultaneously presented to said subject; (f) a processorelectrically coupled to said first sensor and adapted to receive saidfirst potentials and analyze said first potentials to determine if saidfirst potentials comprise data indicative of at least one response tosaid first stimulus; and, (g) the processor, electrically coupled tosaid second sensor and adapted to receive said second potentials andanalyze said second potentials to determine if said second potentialscomprise data indicative of at least one response to said secondstimulus, wherein, each stimulus is presented substantiallysimultaneously.